Issues with 0dBFS+ Levels On Digital Audio Playback Systems - page 4

Audiotrak Prodigy 7.1, analog loopback

How can I not resist testing the soundcard itself? I recorded the soundcard playing the test signals via it's own DACs, and used a loopback cable to send the output back to the input and recorded that (unfortunately only at 44.1kHz/24-bits as the soundcard does not support different sampling rates for input vs output).

As you can see, the soundcard behaved really well, and was able to handle 0dBFS+ levels without clipping:

In fact, I discovered the reason the soundcard was able to handle 0dBFS+ is because it has a built in headroom for mixing purposes - in other words, the volume levels can really go up to 11! (12, actually, but who's complaining)

Conclusion - A Consumer's Perspective on 0dBFS+

I discovered some interesting "facts" about 0dBFS+ whilst writing this article:

• 0dBFS+ levels cannot be generated by recording from an analog source. No matter how high the recording gain level is set at, the digital recording will never contain 0dBFS+ levels. Of course, if the recording gain is set too high, digital "clipping" will occur, but this is not the same thing as 0dBFS+ levels.
• 0dBFS+ levels can be created through subsequent manipulation or processing of a digital recording. For example, if a digital recording is "amplified" (by multiplying each digital sample by a constant amount) it may then subsequently lead to 0dBFS+ levels.
• Therefore, 0dBFS+ levels are technically "illegal" states and represent anomalies or artifacts created by processing signals in the digital domain.
• If a player does not handle 0dBFS+ levels it is NOT strictly speaking the fault of the player. There is no "requirement" in the CD specification for players to be able to handle 0dBFS+ levels and arguably the ability to handle 0dBFS+ levels are a violation of specifications.

So, if 0dBFS+ levels are "illegal" then why should we worry about them? If they represent "mastering errors" then recording engineers shouldn't be creating or releasing titles containing 0dBFS+ levels. Why should manufacturers create workarounds for problems caused in the recording studio?

If only everything was so simple!

First of all, it is not that easy for recording engineers to detect when they have exceeded 0dBFS.

See for example the following link: ( http://www.smr-home-theatre.org/surround2001/technology/page_03.shtml )

for the sort of tools and capabilities that need to be deployed in the recording studio to detect 0dBFS+ levels.

Secondly, many consumers now have access to audio editing and processing tools and therefore it is all too easy for consumers to generate 0dBFS+ levels through a common practice known as "normalization". There is very little awareness or visibility of 0dBFS+ amongst consumers, and the makers of audio tools for consumers are not doing a good job at educating their customers about the dangers of 0dBFS+.

Here are some examples, from help files and user manuals of various tools that I have access to:

n-Track Studio user manual, page 29: "Normalization is the process of amplifying an audio signal so that its maximum amplitude matches the level specified. Normalization can be useful when preparing a wave file for burning onto a CD. Setting the maximum level of all CD tracks to 0 dB assures that no clipping occurs and that the playback level of all tracks is similar"

(So, normalizing everything to 0dB is a good idea then, and I should do it as often as possible? No mention here that it may cause 0dBFS+ levels)

Sony Sound Forge 7.0 user manual, page 160: "As a rule, normalizing using Peak levels to 0dB is acceptable …"

(So, as a rule, it is a Good Thing, no?)

I could go on, but you get the idea. The closest I have seen to an actual warning is the following, taken from the Cool Edit Pro 2.1 (now Adobe Audition ) help file: "If you're planning to put normalized audio on CD, you might want to normalize the waveforms to no more than 96% as some audio compact disc players have problems accurately reproducing bits that have been processed to 100% (maximum) amplitude." This is good, but it does not explain why players will have problems, and may lead me to erroneously conclude that these problems are the fault of the players rather than a problem with normalizing.

Speaking purely as a consumer, the fact of life is, regardless of whose "fault" it is, recordings with 0dBFS+ levels already exist today (or easily created by unsuspecting consumers). I have detected the presence of 0dBFS+ levels in a small but significant portion of my personal CD collection. Sure, I can rant and complain about the incompetence of the recording engineers that created these "faulty" and "illegal" recordings, but that's not going to change the fact that I already own these discs, which I don't intend to sell, and I have no way of forcing the studios to remaster these recordings and re-release them. Well, I could initiate a class action suit, but I suspect that would benefit lawyers more than consumers …

I was surprised to discover in the course of my testing that some manufacturers are aware of the 0dBFS+ issue and have designed players capable of reproducing 0dBFS+ levels without distortion (or at least no more distortion than below 0dBFS levels). The Sony SCD-XA777ES is clearly one such player, and the Panasonic DVD-RP82 has partial support for handling 0dBFS+ levels. Others are either unaware of the issue or is unwilling to make the compromises necessary in order to handle 0dBFS+.

Of course, a player 0dBFS+ levels can create problems further down the chain, as 0dBFS+ levels can overload the preamp stage or compromise the preamp design (It requires a preamp stage that can handle up to 4Vrms instead or 2V).

I would like to salute those manufacturers who have taken the trouble to provide players that handle 0dBFS+. It gives me, as a consumer, the chance to experience "faulty" recordings without the distortion that I would otherwise be forced to hear, provided I am very careful about the choice of equipment I use to play back these recordings.

Many thanks to Christine Tham for this exclusive article.

Visit her very informative website at: http://users.bigpond.net.au/christie/