From: rjs@alice.att.com (Bob Safranek)
Newsgroups: comp.compression
Subject: Re: Too much lossy encoding?
Keywords: PASC, audio
Date: 10 Apr 92 20:01:42 GMT
Reply-To: rjs@alice.att.com
Followup-To: comp.compression
Distribution: comp.compression
Organization: AT&T, Bell Labs


    I guess its time to throw my opinions on this topic into the ring...
    I have been working on perceptual coding of still and moving images for
    last five years and work with people that are working on perceptual audio.

    My definition of perceptual coding is a coding system that uses information
    about the reciever (in these cases the human ear/vision system) to
    adapt the encoding process to concentrate quantization distortion
    (information loss) in portions of the signal that are not percieved
    (filtered out) by the reciever (ear/eye).

    A perceptually lossless coding of a signal is defined as a signal that
    is statistically indistinguishable from the original. This is tested
    using A/B or A/B/X subjective test with a sufficiently large testing
    population.

    I believe it is possible to design coding system's today that can produce 
    perceptually lossless coding of arbitrary audio/video signals PROVIDED
    that the bitrate is allowed to vary with the signal. The "perceptual
    entropy" of a signal can vary greatly, therefore you must allow the
    encoder to generate enough rate to encode what it feels is important,
    but we do not know how to design coders that minimize the percieved
    distortion under all conditions if the bitrate is constrained to be
    less that some factor times perceptual entropy of the signal.
    (The Perceptual Entropy, a term originally proposed for audio
    signals in Johnston, "..Perceptual Entropy..." ICASSP 1988, is
    a very lower bound.)

    The current standards (JPEG, H.261, MPEG (audio and video)) all constrain
    the bitrate to a value that is less the the perceptual entropy of arbitrary
    signals. Therefore, there will be some input signals that will be coded
    to a perceptually lossless level, and some that will not.
    In addition, including MPEG/audio, they do not pay adequate attention to
    modeling the reciever and using that information to adapting the
    quantization of the input.

    There is a place for both lossless and lossy coding in the marketplace.
    Even with fiber to the home, sending multiple HDTV streams (> 1 gigabit/sec)
    to many users will be impractical. The method to use depends on the 
    application at hand. Video teleconferences can get by with lower resolution
    and frame rate (both simple types of lossy coding) that entertainment
    television, which can get by with less than contribution quality video
    (data that will be postprocessed).

    As an example, the current digital HDTV coders that are being tested by
    the FCC compress ~1 GB/sec down to ~15-20 MB/sec. These systems use
    motion compensation (temporal history) and DO NOT produce perceptually
    lossless output for ALL input. They DO produce very good quality on almost
    everything and are nearly lossless on typical input (including MTV type
    special effects). If you raise the bitrate to ~45 MB/sec, you can get
    perceptually lossless coding of all known inputs with the same systems.
    This could be used over fiber to the home systems, and network -> affiliate
    distributions.  If you raise the bitrate to ~135 MB/sec, you can achieve
    the same quality and encode everything as still images (removing the
    motion compensation). This level should be sufficient (and provide the
    needed flexibility) for postproduction and editing (contribution quality).

    Please send responses to me (as well as the net!) since I will be at the
    NAB-HDTV/WORLD (National Association of Broadcasters) conference next 
    week.

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   Bob Safranek                                 rjs@alice.att.com
   AT&T Bell Labs
   Signal Processing Research Dept
   Murray Hill, NJ

   The above are my opinions and may not reflect the opinions of my employer
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