Main Tech Page

© 2017 DIGIFON

From "The ISDN Studio" by Dave Immer
Audio Engineering Society 99th Convention
Oct. 8, 1995 , New York City

"CASCADING" Using Frequency-Domain Coding

This applies to the MPEG and Dolby AC-2 algorithms. If you think about the digital audio bit-reduction process, you will begin to appreciate the hazards and penalties of multiple encode/decode cycles. It can be more severe than the problems of multiple A/D - D/A conversions. At the lower bit-rates of 112/128 Kbps the risk of audio artifacts (noise) accumulating due to sending a stereo waveform through multiple CODEC systems is very high. For this reason it's best to keep cascading to a minimum and avoid doing things like boosting mid & hi-end EQ at the receiving end. But at data bandwidths of 256 Kbps or higher, you can get away with more cascading and post processing of the received stereo audio before noise is perceived.
There is a larger issue here, however, and it deserves careful attention. As the use of these bit-reduction algorithms increases throughout all steps of the audio (and video) production and broadcasting process, the odds of multiple encode/decode cycles also increases. Plus there is a greater chance of "transcoding" from one algorithm to another. To give you an idea of this, here are some of the audio coding applications now, or soon to be, in use.

1. Talent import. This includes voiceovers and multi tracking.
2. Sound Effects, Sample distribution/delivery.
3. Sub mix delivery for editing/post-processing.
4. Final Mix delivery.
5. Surround-Sound systems for movies, laser discs and HDTV.
6. Satellite distribution of Radio and TV programming.
7. Radio STL (studio - transmitter link). In use now by many radio stations.
8. Digital Radio Broadcast. Available in certain test markets now.
9. MiniDisk Recorders. 384kbps or higher.
10. Storage and editing of digital audio on personal computers.

It is quite conceivable that a piece of audio could pass through five or six (or more!) bit-reduction cycles before it arrives at the intended consumers ears. If most of the CODEC cycles were at the relatively low bit rates of 112/128 Kbps stereo, the resulting waveform would quickly accumulate noticeable noise & artifacts. Generally speaking, the way to avoid this waveform deterioration is to use the CODECs at the highest bit-rates possible. For example, extensive testing of the MPEG1 layer 2 algorithm by ISO in the early 90's in Stockholm revealed that a stereo program could withstand 1, perhaps 2, CODEC cycles at 128 Kbps without noticeable artifacts appearing. But at 256 Kbps, listeners didn't hear any deterioration until about 4-5 pass-throughs; and at 384 Kbps, material could withstand 10 or more cycles depending on the waveform complexity. (By contrast, a time-domain algorithm can withstand more CODEC cycles at 384 Kbps before the waveform noticeably deteriorates.) The point here is: Use frequency-domain CODECs carefully. Consider transferring unreduced audio files (SoundDesigner II, etc.) for critical mastering.

"Bridging" or "Hybrid Sessions"

Due the existence of at least 4 popular incompatible algorithms, DIGIFON, EDnet and GlobeCast offer bridging services whereby incompatible users - either independents or competing network subscribers - can both call in to a central station which has both systems on hand, and be simply "patched through" from one to the other. This is a less than elegant solution to system incompatibility, since it requires the waveform to be decoded and "re-coded", thereby adding another layer of bit-reduction to the transmission. (See "CASCADING".) At low bit rates this can be very detrimental to the audio, but if done with care, bridging will yield excellent results. Hybrid session fees add to the cost a transmission. But for all users it greatly expands the number of CODEC systems you can connect to, opening up new markets and talent pools as well as allowing for greater flexibility in recording touring talent and in program delivery.