6 basics of digital audio final

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5/4/2014

Virtual

Environment and

Multimedia System

S M TALHA JUBAED

DEPT. OF COMPUTER SCIENCE ENGINEERING (CSE)

UNIVERSITY OF RAJSHAHI

HOTLINE: + 088- 01911 – 088 706



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Suggestion for CSE-310: Virtual Environment and Multimedia Systems

Question-01: What is motion compensation in video compression? What do you mean by I-Frame, P-

Frame and B-frame? 5 Marks CSE-2009 *** 288 PAGES

Motion Compensation in Video Compression:

Motion compensation is an algorithmic technique employed in the encoding of video data for

video compression, for example in the generation of MPEG-2files. Motion compensation describes apicture in terms of the transformation of a reference picture to the current picture. The reference picturemay be previous in time or even from the future. When images can be accurately synthesised frompreviously transmitted/stored images, the compression efficiency can be improved.

[How it works – ]

Motion compensation exploits the fact that, often, for many frames of a movie, the onlydifference between one frame and another is the result of either the camera moving or an object in theframe moving. In reference to a video file, this means much of the information that represents one framewill be the same as the information used in the next frame.

Using motion compensation, a video stream will contain some full (reference) frames; thenthe only information stored for the frames in between would be the information needed to transformthe previous frame into the next frame.

I-frames: (Intra-Coded Frame):

It is an independent frame that is not related to any other frame.

They are present at regular intervals. I-frames are independent of other frames and cannot be constructed from other frames.

P-frames: (Predicted Frame):

It is related to the preceding I-frame or P-frame.

Each P-frame contains only the changes from the preceding frame. P-frames can be constructed only from previous I- or P-frames.

B-frames: (Bidirectional Frame):

It is relative to the preceding and following I-frame or P-frame. Each B-frame is relative to the past and the future.

A B-frame is never related to another B-frame.

Question-02: What is Water Marking? Briefly discuss the main features that are possessed by watermarks.

5 Marks CSE-2009 ***

Watermarking:

A watermark is a secret code described by a digital signal carrying information about the

copyright property of the product . The watermark is embedded in the digital data such that it isperceptually not visible. The copyright holder is the only person who can show the existence of his own

watermark and to prove the origin of the product.

Reproduction of digital products is easy and inexpensive. In a network environment,like the web, retransmission of copies all throughout the world is easy. The problem of protecting the

intellectual property of digital products can be handled by the notion of watermarks.

Main Features of Water Marking:

Watermarks are digital signals that are superimposed on a digital image causing alternations to the

original data. A particular watermark belongs exclusively to one owner who is the only person that canproceed to a trustworthily detection of the personal watermark and thus, prove the ownership of thewatermark from the digital data. Watermarks should possess the following features:

http://en.wikipedia.org/wiki/Video_compression

http://en.wikipedia.org/wiki/MPEG-2

http://en.wikipedia.org/wiki/Film_frame

http://en.wikipedia.org/wiki/Motion_%28physics%29

http://en.wikipedia.org/wiki/Video_format

http://en.wikipedia.org/wiki/Video_format

http://en.wikipedia.org/wiki/Motion_%28physics%29

http://en.wikipedia.org/wiki/Film_frame

http://en.wikipedia.org/wiki/MPEG-2

http://en.wikipedia.org/wiki/Video_compression



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Perceptual Invisibility:

The modification caused by the watermarks embedding should not degrade theperceived image quality. However, even hardly visible differences may become apparent when theoriginal image is directly compared to the watermarked one.

Trustworthily Detection:

Watermarks should constitute a sufficient and trustworthily part of ownership of a particular product.Detection of a false alarm should be extremely rare. Watermarks signals should be characterized by

great complexity. This is necessary in order to be able to produce an extensive set of sufficiently welldistinguishable watermarks. An enormous set of watermarks prevents the recovery of a particularwatermark by trial-and-error procedure.

Associated Key:

Watermarks should be associated with an identification number called watermark key. The key is usedto cast, detect and remove a watermark. Subsequently, the key should be private and should exclusivelycharacterize the legal owner. Any digital signal, extracted from a digital image, is assumed to be a validwatermark if and only if it is associated to a key using a well established algorithm.

Automated Detection/Search:

Watermarks should combine easily with a search procedure that scans any publicly accessible domain

in a network environment for illegal deposition of an owner’s product.

Statistical Invisibility:

Watermarks should not be recovered using statistical methods. For example, the possession of a greatnumber of digital products, watermarked with the same key, should not be disclose the watermark byapplying statistical methods. Therefore, watermarks should be image dependent.

Multiple Watermarking:

We should be able to embed a sufficient number of different watermarks in the same images. This

feature seems necessary because we cannot prevent someone from watermarking an alreadywatermarked image. It is also convenient when the copyright property is transferred from one owner

to another.

Robustness:

A watermark that is of some practical use should be robust to image modifications up to a certain degree.

The most common image manipulations are compression, filtering, color quantization/color brightnessmodifications, geometric distortions and format change. A digital image can undergo a great deal ofdifferent modifications that may deliberately affect the embedded watermark. Obviously, a watermarkthat is to be used as a means of copyright protection should be detectable up to the point that the hostimage quality remains within acceptable limits.

Question: Write down the Huffman coding algorithm used in lossless compression. Draw the coding tree

for HELLO using the Huffman coding algorithm. 7 Marks CSE-2009 ***

Huffman Coding Algorithm:

1. Initialization: Put all symbols on a list sorted according to their frequency counts.

2. Repeat until the list has only one symbol left:

(a) From the list pick two symbols with the lowest frequency counts. Form a Huffman subtree

that has these two symbols as child nodes and create a parent node.

(b) Assign the sum of the children's frequency counts to the parent and insert it into the listsuch that the order is maintained.(c) Delete the children from the list.



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3.

Assign a codeword for each leaf based on the path from the root.

Coding Tree for HELLOW using the Huffman Algorithm

Figure: Coding Tree for HELLOW using the Huffman Algorithm

Question: What are the advantages of adaptive Huffman coding over Huffman coding? 2 Marks CSE-2012

***

Advantages of adaptive Huffman coding over Huffman coding:

1. The Huffman algorithm requires prior statistical knowledge about the information source andsuch information is often not available. This is particularly true in multimedia applications,where future data is unknown before its arrival as for example in live (or streaming) audio and

video. Even when the statistics are available, the transmission of the symbol table couldrepresent heavy overhead. In adaptive Huffman coding, statistics are gathered and updateddynamically as the DataStream arrives. The probabilities are no longer based on priorknowledge but on the actual data received so far. In adaptive Huffman coding, if the probability

distribution of the received symbols changes, symbols will be given new (longer or shorter)codes. This is especially desirable for multimedia data, when the content (the music or the color

of the scene) and hence the statistics can change rapidly.

2. Adaptive Huffman coding uses defined word schemes which determine the mapping fromsource messages to code words based upon a running estimate of the source messageprobabilities. The code is adaptive and changes so as to remain optimal for the current estimates.In this way, the adaptive Huffman codes respond to locality and the encoder thus learns thecharacteristics of the source data. The decoder must then learn along with the encoder by

continually updating the Huffman tree so as to stay in synchronization with the encoder.

3. A Third advantage of adaptive Huffman coding is that it only requires a single pass over the data.In many cases the adaptive Huffman method actually gives a better performance, in terms of

number of bits transmitted, than static Huffman coding.

Adaptive Huffman codes: respond to locality



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Encoder is "learning" the characteristics of the source. The decoder must learn along with the

encoder by continually updating the Huffman tree so as to stay in synchronization with theencoder.Another advantage: they require only one pass over the data.

Question: Write down the LZW compression algorithm and generate the output code for the string

“ SERURU SE”. The initial dictionary is -1, S-2, E-3, R-4, U-5. What are the limitations of the LZW

compression? 6 Marks CSE-2012 ***

LZW Compression Algorithm:

Second Part:

Let's start with a very simple dictionary (also referred to as a "string table"), initially containing only 3characters, with codes as follows:

Code string-------------------------------------------

1 C2 S3 E

4 R5 U

Now if the input string is “CSERURUCSE”, the LZW compression algorithm works as follows:

S C OUTPUT CODE STRING----------------------------------------------------------------------------------------

1 C

2 S

3 E4 R5 U



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----------------------------------------------------------------------------------------

C S 1 6 CSS E 2 7 SEE R 3 8 ERR U 4 9 RUU R 5 10 URR URU C 9 11 RUCC S

CS E 6 12 CSEE EOF 5

-----------------------------------------------------------------------------------------The output codes are 1 2 3 4 5 9 6 5.

Similar Important Question: Generate the output code for the string “ABABBABCABABBA”. The initial

dictionary is A-1, B-2, C-3.

Let's start with a very simple dictionary (also referred to as a "string table"), initially containing only 3

characters, with codes as follows:

Code string

-------------------------------------------1 C2 S

3 E4 R5 U

Now if the input string is “ABABBABCABABBA”, the LZW compression algorithm works as follows:

The output codes are 1 2 4 5 2 3 4 6 1.



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Limitations of the LZW Compression:

What happens when the dictionary gets too large (i.e., when all the 4096 locations have beenused)?

Here are some options usually implemented:

Simply forget about adding any more entries and use the table as is.

Throw the dictionary away when it reaches a certain size. Throw the dictionary away when it is no longer effective at compression.

Clear entries 256-4095 and start building the dictionary again.

Some clever schemes rebuild a string table from the last N input characters.

Question: Define the 2D Discrete Cosine Transform (DCT). Is DCT is a part of DFT? 2 Marks CSE-2009 ***

2D Discrete Cosine Transform (DCT): Given an input function f(i, j) over two integer variables i and j (a piece of an image), the 2D

DCT transforms it into a new function F(u, v), with integer u and v running over the same range as i andj. The general definition of the transform is:

Second Part: The discrete cosine transform (DCM), a widely used transform coding technique, is able toperform decorrelation of the input signal in a data-independent manner. Because of this, it has gainedtremendous popularity. The DCT is a close counterpart to the Discrete Fourier Transform (DFT). DCT is

likely a transform that involves only the real part of the DFT.

Question: Is DCT is a part of DFT? Justify the answer. 2 Marks CSE-2012 ***

Question: Write down the properties of Huffman coding with example. 3 Marks CSE-2012 ***

Properties of Huffman coding:

The following are important properties of Huffman Coding:

1. Unique Prefix Property: No Huffman code is a prefix of any other Huffman code -precludes anyambiguity in decoding.

2. Optimality: The Huffman code is a minimum redundancy code. It has been proved optimal for agiven data model (i.e., a given, accurate, probability distribution):

– The two least frequent symbols will have the same length for their Huffman codes, differingonly at the last bit.

– Symbols that occur more frequently will have shorter Huffman codes than symbols that occurless frequently.

– The average code length for an information source S is strictly less than η+ 1. Combined with

Eq. ( η ≤ )̅, we have:

η ≤ ̅ < η + 1

Question: What is the entropy of an information source with alphabet = { , , … … … . } ? 2 Marks

CSE-2012 ***

Entropy of an Information source:

The entropy η of an information source with alphabet = { , , … … … . }is:



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Where is the probability that symbol in S will occur.

The term

indicates the amount of information contained in , which corresponds to the number

of bits needed to encode .

Question: What do you mean by JPEG? Write down the main steps required for JPEG encoder and draw

the block diagram for JPEG encoder. 8 Marks CSE-2009 ***

JPEG:

JPEG is an image compression standard that was developed by the "Joint Photographic ExpertsGroup". JPEG was formally accepted as an international standard in 1992. JPEG is a lossy image

compression method. It employs a transform coding method using the DCT (Discrete CosineTransform).

An image is a function of i and j (or conventionally x and y) in the spatial domain. The 2D DCTis used as one step in JPEG in order to yield a frequency response which is a function F (u, v) in thespatial frequency domain, indexed by two integer’s u and v.

Main steps required for JPEG encoder: The main steps required for JPEG encoder are as follows:

Transform RGB to YIQ or YUV and subsample color.

Perform DCT on image blocks.

Apply Quantization.Perform Zig-zag ordering and run-length encoding.

Perform Entropy coding.

The block diagram for JPEG encoder:



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Question: Write short notes on the following topics: Progressive JPEG CSE-2011

Progressive JPEG: The JPEG standard supports numerous modes (variations). Some of the commonly used ones are:

1.

Sequential Mode2. Progressive Mode3.

Hierarchical Mode4.

Lossless Mode

Progressive JPEG delivers low quality versions of the image quickly, followed by higher quality passesand has become widely supported in web browsers. Such multiple scan of images are of course usefulwhen the speed of the communication line is low. In progressive mode, the first few scans carry only afew bits and deliver a rough picture of what is to follow. After each additional scan, more data is receivedand image quality is gradually enhanced. The advantage is that the user-end has a choice whether tocontinue receiving image data after the first scan.

Progressive JPEG can be realized in one of the following two ways:

1. Spectral selection: This scheme takes advantage of the "spectral" (spatial frequency spectrum)characteristics of the DCT coefficients: higher AC components provide detail information.

Scan 1: Encode DC and first few AC components, e.g., AC1, AC2.Scan 2: Encode a few more AC components, e.g., AC3,AC4, AC5.

.

.

.Scan k: Encode the last few ACs, e.g, AC61,AC62, AC63.

2. Successive approximation: Instead of gradually encoding spectral bands, all DCT coefficients are

encoded simultaneously but with their most significant bits (MSBs) first.

Scan 1: Encode the first few MSBs, e.g., Bits 7, 6, 5, 4.Scan 2: Encode a few more less significant bits, e.g., Bit 3.

.

.

.Scan m: Encode the least significant bit (LSB), Bit 0.

Question: Briefly discuss the parameters measured in Quality of services for multimedia data

transmission. 4 Marks CSE-2011 CSE-2009 ***

Parameters measured in Quality of services for multimedia data transmission: Quality of Service (QoS) for multimedia data transmission depends on many parameters. Some of the

most important are:

Data rate: A measure of transmission speed, often measured in kbps or Mbps.

Latency (maximum frame/packet delay): maximum time needed from transmission to reception,often measured in milliseconds(msec).For example, when the round-trip delay exceeds 50 msec, echo becomes a noticeable problem;when the one way delay is longer than 250 msec, talker overlap will occur, since each talker will

talk without knowing the other is also talking.

Packet loss or error: A measure (in percentage) of error rate of the packetized data transmission.Packets get lost or garbled, such as over the internet. They may also be delivered late or in the



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wrong order. Since retransmission is often undesirable, a simple error-recovery method for real-

time multimedia is to replay the last packet hoping the error is not noticeable.

In general, for uncompressed audio/video, packet loss is < 10−. For compressed multimedia

and ordinary data, the desirable packet loss is less than 10−7 10−.

Jitter (or delay jitter): A measure of smoothness of the audio/video playback. Technically, jitter

is related to the variance of frame/packet delays. A large buffer (jitter buffer) can to hold enoughframes to allow the frame with the longest delay to arrive, to reduce playback jitter. However,this increases the latency and may not be desirable in real-time and interactive applications.

Sync skew: A measure of multimedia data synchronization, often measured in milliseconds(msec). For a good lip synchronization, the limit of the sync skew is ± 80 between audioand video. In general ±200 is still acceptable.

Question: When Real Time Streaming Protocol (RTSP) is used? Briefly discuss its operation. 4 Marks CSE-

2009 ***

When Real Time Streaming Protocol (RTSP) is used:

Streaming Audio and Video are Audio and video data that are transmitted from a stored media server tothe client in a data stream that is almost instantly decoded. ( )

The operations of Real Time Streaming Protocol (RTSP): RTSP is for communication between a client and a stored media server. Fig. 16.5 illustrates a possiblescenario of four RTSP operations:

1. Requesting presentation description: the client issues a DESCRIBE request to the Stored MediaServer to obtain the presentation description such as media types (audio, video, graphics etc),

frame rate, resolution, codec, etc. from the server.

2.

Session setup: the client issues a SETUP to inform the server of the destination IP address, portnumber, protocols, TTL (for multicast). The session is set up when the server returns a session

ID.

3. Requesting and receiving media: after receiving a PLAY, the server started to transmit streamingaudio/video data using RTP. It is followed by a RECORD or PAUSE. Other VCR commands, such

as FAST-FORWARD and REWIND are also supported. During the session, the client periodicallysends an RTCP packet to the server, to provide feedback information about the QoS received.

4. Session closure: TEARDOWN closes the session.



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Question: Describe the SIP Protocol In Internet Telephony. 6 Marks CSE-2011

SIP Protocol in Internet Telephony:

SIP is an application-layer control protocol in charge of the establishment and termination ofsessions in Internet telephony. These sessions are not limited to VoIP communications- they also includemultimedia conferences and multimedia distributions. SIP is a text-based protocol, also a client-serverprotocol that is different from H.323. SIP can advertise its session using email, news group, web pages

or directories, or SAP — a multicast protocol.

The methods (commands) for clients to invoke:

INVITE: invites callee(s) to participate in a call.

ACK: acknowledges the invitation.

OPTIONS: enquires media capabilities without setting up a call.

CANCEL: terminates the invitation.

BYE: terminates a call.

REGISTER: sends user's location info to a Registrar (a SIP server).



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Scenario of a SIP Session

Fig. 16.7 illustrates a scenario when a caller initiates a SIP session:Step 1. Caller sends an "INVITE [email protected]" to the local Proxy server P1.Step 2. The proxy uses its DNS (Domain Name Service) to locate the server for [email protected] and sends the request to it.Step 3,4. [email protected] is current not logged on the server. A request is sent to the nearbylocation server. John's current address [email protected] is located.Step 5. Since the server is a Redirect server, it returns the address [email protected] to the Proxy

server P1.Step 6. Try the next Proxy server P2 for [email protected]. Step 7,8. P2 consults its Location server and obtains John's local address john [email protected]. Step 9,10. The next-hop Proxy server P3 is contacted, it in turn forwards the invitation to wherethe client (callee) is.Step 11-14. John accepts the call at his current location (at work) and the acknowledgments arereturned to the caller.

Question: Write short notes on the following topics: RSVP CSE-2011

RSVP (Resource ReserVation Protocol)

RSVP is a setup protocol for internet resource reservation. RSVP is developed to guarantee desirable

QoS, mostly for multicast although also applicable to unicast.

A general communication model supported by RSVP consists of m senders and n receivers, possibly in

various multicast groups (e.g. Fig.16.4(a)). The most important messages of RSVP:1. Path Message: A Path message is initiated by the sender, and contains information about the

sender and the path (e.g., the previous RSVP hop).2. Resv message: A Resv message is sent by a receiver that wishes to make a reservation.

Main Challenges of RSVP

a. There can be a large number of senders and receivers competing for the limited networkbandwidth.

b. The receivers can be heterogeneous in demanding different contents with different QoS.

c.

They can be dynamic by joining or quitting multicast groups at any time.

mailto:[email protected]














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Figure 16.4: A Scenario of network resource reservation with RSVP.

Fig. 16.4 depicts a simple network with 2 senders (S1, S2), three receivers (R1, R2, and R3) and 4 routers(A, B, C, D):

1.

In (a), Path messages are sent by both S1 and S2 along their paths to R1, R2, and R3.2.

In (b) and (c), R1 and R2 send out Resv messages to S1 and S2 respectively to make reservationsfor S1 and S2 resources. Note that from C to A, two separate channels must be reserved since R1and R2 requested different data streams.

3.

In (d), R2 and R3 send out their Resv messages to S1 to make additional requests. R3's request

was merged with R1's previous request at A and R2's was merged with R1's at C.

Question: What do you mean by MIDI message? Discuss different types of MIDI message. 4 Marks CSE-

2011 ***

MIDI Message:

MIDI messages are used by MIDI devices to communicate with each other.Structure of MIDI messages:

MIDI message includes a status byte and up to two data bytes.Status byte

o The most significant bit of status byte is set to 1.o The 4 low-order bits identify which channel it belongs to (four bits produce 16 possible

channels).o The 3 remaining bits identify the message.

The most significant bit of data byte is set to 0.

Different types of MIDI message:



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Channel messages:

A Channel message can have up to 3 bytes: The first byte is the status byte (the opcode, as it

were); has its most significant bit set to 1.The 4 low-order bits identify which channel this message belongs to (for 16 possible channels).The 3 remaining bits hold the message. For a data byte, the most significant bit is set to 0.

Voice messages:

This type of channel message controls a voice, i.e., sends information specifying which note toplay or to turn off, and encodes key pressure.Voice messages are also used to specify controller effects such as sustain, vibrato, tremolo, andthe pitch wheel. Table 6.3 lists these operations.

Table 6.3: MIDI Voice Message

Channel mode messages:

Channel mode messages form a special case of the Control Change message and therefore allmode messages have opcode B (the message is &HBn, or 1011nnnn).However, a Channel Mode message has its first data byte in 121 through 127 (&H79–7F).Channel mode messages determine how an instrument processes MIDI voice messages.Some examples include respond to all messages, respond just to the correct channel, don'trespond at all, or go over to local control of the instrument.The data bytes have meanings as shown in Table 6.4.

System Messages:



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System messages have no channel number and are meant for commands that are not channel

specific, such as timing signals for synchronization, positioning information in prerecorded MIDIsequences, and detailed setup information for the destination device.

Opcodes for all system messages start with “&HF”.

System messages are divided into three classifications, according to their use:1.

System common messages 2. System real-time messages 3.

System exclusive messages

System Common Message: Table 6.5 sets out these messages, which relate to timing or positioning..

Table 6.5: MIDI System Common Message

System Real-Time Messages: Table 6.6 sets out system real time messages, which are related to

synchronization

Table 6.6: MIDI system real time messages.

System exclusive messages:

System exclusive messages is included so that the MIDI standard can be extended by

manufacturers.After the initial code, a stream of any specific messages can be inserted that apply to their ownproduct.

A System Exclusive message is supposed to be terminated by a terminator byte “&HF7”, asspecified in Table 6.5

The terminator is optional and the data stream may simply be ended by sending the status byteof the next message.

Question: Define Multimedia System. What are the desirable features of a multimedia system? 5 Marks

CSE-2012 CSE-2011 CSE-2010 ***

Multimedia System:

A Multimedia System is a system capable of processing multimedia data and

applications. A Multimedia System is characterised by the processing, storage, generation, manipulationand rendition of Multimedia information.

Characteristics of a Multimedia System

A Multimedia system has four basic characteristics:



1.

Multimedia systems must be computer controlled.

2. Multimedia systems are integrated.3. The information they handle must be represented digitally.4. The interface to the final presentation of media is usually interactive.

Desirable Features for a Multimedia System

The following feature is desirable (if not a prerequisite) for a Multimedia System:1.

Very High Processing Power — needed to deal with large data processing and real time deliveryof media. Special hardware commonplace.

2. Multimedia Capable File System —needed to deliver real-time media — e.g. Video/AudioStreaming.

3. Special Hardware/Software needed – e.g. RAID technology.4. Data Representations — File Formats that support multimedia should be easy to handle yet

allow for compression/decompression in real-time.5. Efficient and High I/O —input and output to the file subsystem needs to be efficient and fast.

Needs to allow for real-time recording as well as playback of data. e.g. Direct to Disk recordingsystems.

6. Special Operating System —to allow access to file system and process data efficiently and

quickly. Needs to support direct transfers to disk, real-time scheduling, fast interrupt processing,I/O streaming etc.

7.

Storage and Memory — large storage units (of the order of hundreds of Tb if not more) and large

memory (several Gb or more). Large Caches also required and high speed buses for efficientmanagement.

8.

Network Support — Client-server systems common as distributed systems common.

9. Software Tools — user friendly tools needed to handle media, design and develop applications,deliver media.

Question: What do you mean by multimedia authoring? What are the functions of the authoring tools?

Name some multimedia authoring systems/ tools. 2 Marks CSE-2012 ***

Multimedia authoring:

Multimedia authoring involves assembling, arranging and presenting information in the

structure of a digital multimedia, which can include text, audio, as well as moving images. This processrequires a tool known as author ware, a program that helps in writing hypertext or multimediaapplications. Tools that provide are the capability for creating a complete multimedia presentation,including interactive user control, are called authoring programs.

Multimedia Authoring Tools:

Macromedia Flash: allows users to create interactive movies by using the score metaphor, i.e., atimeline arranged in parallel event sequences.

Macromedia Director: uses a movie metaphor to create interactive presentations — very

powerful and includes a built‐in scripting language, Lingo, that allows creation of complexinteractive movies.

Authorware: a mature, well‐supported authoring product based on the Iconic/Flow‐control

metaphor.

Quest: similar to Authorware in many ways, uses a type of flowcharting metaphor. However, theflowchart nodes can encapsulate information in a more abstract way (called frames) thansimply subroutine levels.

6 basics of digital audio final

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