From: rowlands@ra.ti.com (Jon Rowlands)
Subject: Re: Too much lossy encoding?
Keywords: PASC, audio
Organization: Texas Instruments Computer Science Laboratory, Dallas
Date: Thu, 9 Apr 1992 02:47:00 GMT

Apologies for my previous posting - a slip of the mouse. Let me try that
again...

In article <1992Apr8.160300.5981@tc.fluke.COM>, kurt@tc.fluke.COM (Kurt
Guntheroth) writes:
|> Is it just me or is anyone else out there concerned that we are using lossy
|> encoding "too much" as a substitute for denser storage and better
|> transmission
|> methods?
|> 
|> Analog recording and storage methods are inherently lossy, but the loss
|> depends on the quality of the recording and playback mechanisms.  As
|> mechanisms improve, the quality of the recorded, stored, and played
|> information
|> improves.
|> 
|> Digital recording and storage methods have a fixed quality that depends on
|> the way the data was encoded.  Once a format has been selected, no further
|> gain in quality is possible, only reduction in the effects of aging on the
|> digital bits.
|> 
|> Digital storage mechanisms like audio CD's force us to accept a (Nyquist)
|> limitation on the stored data, even though they encode the full imput
|> waveform.  While the conventional wisdom is that we cannot hear sound above
|> 22 KHz, this is not completely true; we are sensitive to phase changes at
|> higher frequencies.  Now proposals like PASC and audio MPEG are coming
|> along
|> that are lossy; they filter out sounds "we can't hear anyway".  We are
|> moving
|> down from being "about as good as a phonograph" to "about as good as FM
|> radio"
|> to achieve playback on media that are still undergoing rapid development.
|> 
|> It's just the same in the video world; complex formats based on removing
|> visual data "we can't perceive anyway" are multiplying.  Only it isn't
|> always true.  There are problem images, like sports and other fast motion
|> subjects.  
|> 
|> Ten or twenty years from now, when storage densities are far higher,
|> transmission facilities have more bandwidth (for instance fiber optic
|> cable to your phone and TV), and better picture tubes and amplifiers have
|> appeared (as they always have before), aren't we going to be really pissed
|> off that we accepted so much lossy encoding that we can in fact detect?
|> I hope everybody implementing this technology will think about this.  You
|> are going to have to live with your decisions.  

This is an interesting point, but I believe that in the situations you
describe
lossy compression is still the method of choice. I am currently implementing
MPEG audio coders in software, so I've had some chance to observe the behavior
of this and related algorithms. I'd like to respond with a few comments.

1. Concerning the current technology, and MPEG audio in particular. The
"lossy"
model works extremely well for useful compression factors, say 3:1 or 4:1.
There
is literally no audible difference between the uncompressed original and the
decoded sequence. There is no question of settling for "about as good as FM
radio". Audio signals that cause the algorithm to perform badly also are rare
(well, I haven't found one yet, and I've been trying). The only assumption
made
about the signal is that it is intended for human listening. Higher
compression
ratios allow quality to be traded for other benefits like low cost, and the
mathematical and psycho-acoustic basis for lossy coding is intended to ensure
that the quality is reduced as little as possible. Nobody says you have to
take
this option, but if "the market" wants it, it's there.

2. Future standards. Efforts ARE being made to allow for future technology in
current standards. The AES consumer and professional standards for digital
audio equipment signal formats allow an audio sample rate of 48kHz (i.e. 24kHz
bandwidth) and sample resolutions up to 20 something (sorry) bits. These
parameters well surpass the capability of the human ear to fault. Note that
compression technology will allow this performance to be had at reasonable
cost far sooner than if you just wait for increased media capacity. DAT is not
a valid counter example - although it uses 48kHz sampling rate without
compression, the penalty is complex mechanics which make it bulky and
expensive with little chance of significant cost reduction or expansion to
beyond 16 bit sample resolution. This merely illustrates the point.

3. Forwards prediction. All the lossy audio, image and video compression
schemes such as JPEG, MPEG audio and video and CCITT H.261 are forward
predictive systems, meaning that the encoder alone makes the decision about
which components of the signal are imperceptible. This inherently allows
for full forwards and backwards compatibility with future improvements in
compression algorithms and computationally more powerful encoders. So even
though current video encoders are not good enough to transparently encode
all images, as you point out, the mechanism is in place for their descendants
do so and interwork painlessly with current systems.

4. Economic factors. Face it, we're NEVER going to have "enough" bandwidth
no matter how far technology advances - we'll always think of something else
to use it for. Perceptually transparent lossy coding allows increased media
capacity to be used for more tangibly useful purposes. Such as multiple audio
channels for "surround" sound, multi-lingual speech, auxiliary data channels,
longer playing times, cost reduction, and a bundle of other things someone
will become rich discovering.

5. Evolution - punctuated equilibrium? Standards and formats that provide for
the future are vital in an environment where advances occur often, I agree
totally. But we can only predict the future when it looks pretty much like
the past. Eventually something changes radically and old standards become
inappropriate and restrictive, such as has occurred with vinyl LP records,
leaded gasoline and cordouroy flares. Instead of burdening ourselves with
the overhead of technology we can't afford or use for an indefinite time,
it seems better (to me) to do what needs to be done now and tomorrow and
then do our best for the future we only imagine we see. Carpae diem.

There is a clear need for lossless storage and tranmission in audio and video,
of course. Few of the standards can interwork without additional loss, and
none are idempotent. So whenever a signal is to be used as input to another
system instead of just reproduced for humans, lossless or no compression
should probably be used.

So now you've found me out. I only said it was a good point because it gave
me the opportunity to rant a bit. Thanks for listening anyway,
Jon
-- 
  _  ,                                               _
 / `- \ Jon Rowlands         phone: 1-214-995-3436 _| "--_ People say I sound
 \_--_/ rowlands@ra.csc.ti.com fax: 1-214-995-0304 \_  __/ like a corporation,
     ~ `-> Texas Instruments CSL, Dallas, TX <-?!-'  \_|  but I ain't no body.