Introduction tMyn 1

Introduction

• MPEG, Moving Picture Experts Group was started in 1988 as a working group within ISO/IEC with the aim of defining standards for digital compression of audio-visual signals.

• MPEG’s first project, MPEG-1 was published in 1993 as ISO/IEC 11172.

• In general MPEG is asymmetrical process: it has been estimated that encoding needs 100 times more processing power compared to the decoding process.

Introduction tMyn 2

• MPEG-1 consists of six parts:

system ISO/IEC 11172-1video ISO/IEC 11172-2audio ISO/IEC 11172-3low bit rate audio ISO/IEC 11172-4conformance testing ISO/IEC 11172-5simulation software ISO/IEC 11172-6

Introduction tMyn 3

• ”Coding of moving pictures and associated audio for digital storage media”

• The original goal was playback of digital audio and video using a standard CD with a bit rate of 1.416 Mbits/sec.

• MPEG 1 was designed to handle progressive (also referred to as noninterlaced) video.

• MPEG1 uses a family of three audio coding schemes, called Layer 1, Layer 2 and Layer 3, with increasing complexity and sound quality.

• The three layers are hierarchical: a Layer 3 decoder handles Layers 1, 2 and 3 and so on…

Introduction tMyn 4

• All layers support 16-bit digitized audio using 16 (only MPEG 2), 22.05 (only MPEG 2), 24 (only MPEG 2), 32, 44.1 or 48 kHz sampling rates.

• Layers work typically with bit rates:– Layer1: 32-448 kbits/sec– Layer 2: 8-384 kbits/sec– Layer 3: 8-320 kbits/sec

• Two audio channels are supported with four modes of operation:– normal stereo– joint(intensity and/or ms) stereo– dual channel mono– single channel mono

Introduction tMyn 5

• For normal stereo, one channel carries the left audio signal and one channel carries the right audio channel.

• For intensity stereo (supported by all layers) high frequencies (above 2 kHz) are combined. The stereo image is preserved but only the temporal envelope is transmitted.

• For ms stereo (supported by Layer 3), one channel carries the sum signal (L+R) and the other the difference (L-R) signal.

Introduction tMyn 6

• To determine which layer should be used for a specific application, look at the available bit rate, as each layer was designed to support certain bit rates with a minimum degradation of sound quality.

• Layer 1, a simplified version of Layer 2, has a target bit rate of about 192 kbits/sec per channel or higher.

• Layer 2 is identical to MUSICAM, and has a target bit rate of about 128 kbits/sec per channel.– Masking-pattern adapted Universal Subband Integrated

Coding and Multiplexing

Introduction tMyn 7

• Layer 3 (also known as mp3) merges the best ideas of MUSICAM and ASPEC and has a target bit rate of about 64 kbits/sec per channel.– Adaptive Spectral Perceptual Entropy Coding

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• All layers use a coding scheme based on psychoacoustic principles – in particular, ”masking” effect where, for example, a loud tone at one frequency prevents another, quieter, tone at a nearby frequency from being heard.

• Adjusting the noise floor reduces the amount of needed data, enabling further compression.

• CDs use 16 bits of resolution to achieve a signal-to-noise ratio (SNR) of about 96 dB, which just happens to mach the dynamic range of hearing pretty well.

Introduction tMyn 9

• If 8-bit resolution were used, there would be a noticeable noise during silent moments in the music.

• However, noise isn’t noticed during loud passages due to the masking effect, which means that around a strong sound you can raise the noise floor since the noise will be masked anyway.

• For a stereo signal, there usually is redundancy between channels.

Introduction tMyn 10

• Table 1 lists some of the constrained parameters.• The CPB (Constrained Parameters Bitstream) is a

limited set of sampling and bit rate parameters designed to standardize buffer sizes and memory bandwidths, allowing a nomimal guarantee of interoperability for decoders and encoders, while still addressing the widest possible range of applications.

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horizontal resolution

vertical resolution

picture area,number of macroblocks

picture rate

pel rate,number of macroblocks/s

bit rate

max 768 samples (pixels)

max 1,856 Mbps

max 576 scan lines

max 396 macroblocks

max 396*25 macroblocks per second

max 30 frames per second

Table 1. Some of the constrained parameters for MPEG 1.

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• The CPB limits video to 396 macroblocks (396*16*16 = 101376 pixels).

• Therefore, MPEG 1 video is typically coded at SIF resolutions of 352*240 or 352*288.

• SIF, Standard Input Format:352 (horizontal)*240 (vertical)*29,97 (fps), NTSC352 (horizontal)*288 (vertical)*25,00 (fps), PAL

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• MPEG 1 video data uses the 4:2:0 format shown in Figure 1.

YC Cb r

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Active linenumber

3

4

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2

1

6

= Y sample

= Calculated Cb, Cr sample

Figure 1. Sampling for 4:2:0 MPEG 1.YC Cb r

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• MPEG 2, ”Generic coding of moving pictures and associated audio information” , extends MPEG 1 to cover a wider range of applications.

• The primary application targeted during the definition process was all-digital transmission of broadcast-quality video at bit rates of 4-9 Mbits/sec.

• MPEG 2 is an ISO/IEC 13818 standard, and consists of nine parts:

Introduction tMyn 16

systems ISO/IEC 13818-1video ISO/IEC 13818-2audio ISO/IEC 13818-3conformance testing ISO/IEC 13818-4software simulation ISO/IEC 13818-5DSM-CC extensions ISO/IEC 13818-6 (Digital Storage Media Command and Control)advanced audio coding ISO/IEC 13818-7 (Dolby Digital 5.1)RTI extension ISO/IEC 13818-9 (Real Time Interface)DSM-CC conformance ISO/IEC 13818-10

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• Figure 1 illustrates the positioning of samples for the 4:2:0 format.

• 4:2:0 implements a 2:1 reduction of and in both the vertical and horizontal directions.

• To display 4:2:0 data, it is first converted to 4:4:4 data, using interpolation to generate the new and samples.

• Actually MPEG 2 4:2:0 is not same as MPEG 1, see the spatial relationships between luminance and color difference samples.

YC Cb r

YC Cb r C b C r

YC Cb rYC Cb rC b C r

Introduction tMyn 18

Active linenumber

3

4

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= Y sample

= Calculated Cb, Cr sample

Figure 1. 4:2:0 sampling forMPEG 2. The sampling positions on the active scan lines of a progressive or noninterlaced picture.

YC Cb r

Introduction tMyn 19

• Figure 2 illustrates the positioning of samples for 4:2:2 format.

• For every two horizontal Y samples, there is one and one sample.

• Each sample is typically 8 bits (consumer applications) or 10 bits (pro-video applications) per component.

• Each sample therefore requires 16 bits (or 20 bits for pro-video applications), usually formatted as shown in Figure 3.

YC Cb r

C bC r

Introduction tMyn 20

Active linenumber

3

4

5

2

1

6

= Y sample

= Calculated Cb, Cr sample

Figure 2. 4:2:2 sampling for MPEG 2. The sampling positions on the active scan lines of a progressive or noninterlaced picture. Co-sited sampling.

YC Cb r

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Y7-0Y6-0Y5-0Y4-0Y3-0Y2-0Y1-0Y0-0

CB7-0CB6-0CB5-0CB4-0CB3-0CB2-0CB1-0CB0-0

SAMPLE0

SAMPLE1

SAMPLE2

SAMPLE3

Y7-1Y6-1Y5-1Y4-1Y3-1Y2-1Y1-1Y0-1

Y7-2Y6-2Y5-2Y4-2Y3-2Y2-2Y1-2Y0-2

Y7-3Y6-3Y5-3Y4-3Y3-3Y2-3Y1-3Y0-3

CB7-2CB6-2CB5-2CB4-2CB3-2CB2-2CB1-2CB0-2

CR7-0CR6-0CR5-0CR4-0CR3-0CR2-0CR1-0CR0-0

CR7-2CR6-2CR5-2CR4-2CR3-2CR2-2CR1-2CR0-2

Figure 3. Frame buffer formatting in MPEG 2 4:2:2.

-0 = SAMPLE 0 DATA-1 = SAMPLE 1 DATA-2 = SAMPLE 2 DATA-3 = SAMPLE 3 DATA

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• For broadcast applications sufficient chrominance resolution can be provided with the 4:2:0 sampling.

• This method of sampling is generally accepted as the most applicable level and profile for MPEG 2, and is termed Main Profile at Main Level or MP@ML.

• Figure 4 illustrates MPEG 2 4:2:0 sampling in Digital TV environment.

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Active linenumber

[4]

5

7

3

1

[8]

= Y sample

= Calculated Cb, Cr sample

Figure 4. Sampling 4:2:0 for MPEG 2. The sampling positions onthe active scan lines of an interlaced picture (top_field_first = 1).

YC Cb r

[2]

[6]

[] = Field N+1

FIELD N FIELD N+1