| Audio encoding is
essential, especially with the limited bandwidth of the Internet and the
likes, or perhaps there is just a need to access more information within the
limited bandwidth. It good to know that efforts are still continuing to
increase the bandwidth, as well as to improve the compression algorithms
without losing or sacrificing audio quality.
On 31st August, 2001, about 40
participants gathered at the Singapore Subordinate Court Auditorium
to listen to a presentation on MPEG 4 AAC (Advanced Audio Coding) given by
Mary Ann Seidler, the International Director of Sales, Telos
System/Omnia based in USA.
After a brief introduction by
Robert Soo, the Chairman of the AES Singapore Section, Mary
Ann Seidler began her presentation with a brief history of various
compression methods like MPEG 1 layer 1, 2 and 3. As the layer number
increases, so does the respective implementation complexities and
performance. It was also noted that the all too familiar MP3 is actually
MPEG 1 Layer 3 and not MPEG 3, as there is no such standard as MPEG 3, as
many would have mistaken. MPEG 1 Layer 3 is also widely used in broadcast
Codecs.
Mary Ann proceeded to the
objective of MPEG 4 AAC. It is designed to provide performance superior to
any known approach at bit rates greater than 64kbps and an excellent
alternative at bit rates reaching as low as 16kbps. MPEG 4 can also work
with sampling rates from 8khz to 96khz, bit rates from 16kbits/s to
576kbits/s, and from 1 to 48 channels.
Researchers are convinced that by
abandoning backward compatibility to earlier MPEG layers, they could make
significant improvements to make the compression algorithm provide
indistinguishable quality at 64kbits/s per mono channel.
The audio test material they chose
are among the most difficult to encode known to Codec developers, isolated
pith pipe, harpsichord, glockenspiel and others. If the Codec is able to
handle these efficiently, than normal audio program like voice and pop music
would certainly perform well too.
Here are some benefits compared to
previous MPEG layers:
- An improved filter bank with a
frequency resolution of 2048 spectral components. Layer 3 has 576;
- A new powerful element called
Temporal Noise Shaping, which is good for voice signals;
- Prediction;
- Perceptual Noise Shaping, which
allows for more efficient use of bits;
It also has some of the Layer 3
features:
- Redundancy Reduction, a completely
loss-less process that encode more frequently appearing values with short
words and rarely appearing values with longer words;
- Bit reservoir buffering. With
portions of music which are more difficult to encode at a given bit-rate
without audible noise, it’ll require a higher bit rate. Similarly, parts of
music which are easier to encode will require less bits. So the extra bits
are stored in the reservoir to be used where it is needed more. This will
improve the audio quality without increasing the bit-rate;
- Ancillary Data. The bit
reservoir buffer offers a solution to include ancillary data like text or
control signalling;
- The Joint Stereo Mode. This
takes advantage of the redundancy in the stereo program material;
With all these features, MPEG 4
AAC is able to achieve indistinguishable high quality audio at 128 kbits/s
per stereo channel. This is actually a 30% or more improvement over MPEG 1
Layer 3 or MP3. To the layperson, this would mean better quality or CD
quality audio, with a file size smaller than that of MP3. Now it’s possible
to put a whole sound effect collection onto a hard disk.
Moving on to a time for questions
and answers, many questions relating to compression algorithms, availability
of software encoders, players and comparison to Mini-Disc were brought up by
the enthusiastic audience.
The discussion continued till
about 9.30pm, after which Robert Soo presented an AES Singapore
Section plaque of appreciation to Mary Ann for such an interesting
presentation. The section would also like to appreciate Mr Richard Lau
of the Singapore Subordinate Court for allowing the Singapore Section to
hold another successful event at their premise. |
