Need advice on saving/mastering files for eventual Audiobook streaming via Audible

Question

I've been producing audiobooks for Audible for 3+ years now.

Mostly, I don't get a chance to listen back to what customers are hearing through the audible app, but just recently I had to check a production to make sure a change had gone live.

I was quite shocked by the audio quality - or rather lack of!

I already knew that audio is down-sampled by Audible for streaming and was given to understand this was 64kbps @22050 Hz . I always check my finished audio at this rate to make sure it sounds OK.

Here's an example: https://drive.google.com/file/d/1pjFmCfS4234eijvzMFsWveaPQmKSt18w/view?usp=sharing

Here's the same passage, stream recorded from Audible's web-based app: https://drive.google.com/file/d/1k5ZmZ_4_DMS_MOtDlwHoztg3WEGjPLNP/view?usp=sharing

My main concern are the high-frequency artifacts in the audible stream... I'm not quite sure how to describe them but it's like a gated hiss that corresponds to the speech.

I'm fairly sure this is a result of audio compression codecs used for the audible stream. From what little I know, I think Audible uses their own .aa format for streaming.

Here's where it gets interesting...

It seems not all streams are "equal" as far as Audible are concerned.

Here's a sample of another narrator/producer who I would loosely class as a contemporary of mine, again recorded from the Audible stream: https://drive.google.com/file/d/1JFPyIPGfZa48NUPwQLk5YrwbHZxWC2PF/view?usp=sharing

- similar quality/artifacts

Here's John Banks reading from "The Time Machine" by H G Wells, which you might say is more "premium" content from Audible's point of view: https://drive.google.com/file/d/1fAkhMOMbWbXOsf_18BjJFqYv9cUTw0YZ/view?usp=sharing

-none of the high-frequency artifacts, but still clearly downsampled (quite close to my first sample).

And finally, Here's a sample for the audio stream of Stephen Fry's "Victorian Secrets" series - "front-page" content on Audible: https://drive.google.com/file/d/1ueak4dpILjpM-pYqoKeiB2PF1kgzPUFp/view?usp=sharing

-crystal clear! (incidentally, this content took longer to buffer than the other examples)

So... it looks like I'm stuck with the lowest-quality stream for my content on Audible, which leads me to wonder if there's anything I can do technically within Audition when mastering/encoding the files in order to maybe reduce if not eliminate those nasty high-frequency artifacts?

My files are submitted at 44100 16bit 192kbps mp3s as per Audible/ACX specification. They happen also to be stereo due to my occasional use of sound effects and/or music, as well as the mastering plugins I use sounding nicer in stereo anyway.

Would appreiate any advice!

SteveG_AudioMasters_ · Answer

This is what I think might be going on... It's difficult with MP3 files to get a clue, but there is one. And that's the noise floor. In your example, it's at about -48dB and in the Stephen Fry example, the only bit I could find was at a much lower level, at about -78dB. And indeed in your original sample, the noise floor is higher. This on its own may well account for the static-like sounds - the noise floor is getting much closer to 'clearly audible'. I did wonder whether they were resampling, but I don't think so - that would introduce other artifacts and generally make things a lot worse. So my guess is that you need to rearrange your signal processing so that the resultant noise floor is more like the Stephen Fry example, whilst remaining within the ACX gamut. The fact that they are stereo files should make no difference, incidentally. In order not to introduce other problems, you might need to be a bit subtle about this - you don't want to downward-expand the noise floor within the dynamic range of your audio, because that will make everything appear to be disappearing into a bottomless pit. But expanding out the bottom part of your signal from the level of the quietest point down to a similar level as the Stephen Fry example might go a long way. The ultimate noise floor of a 16-bit file is much lower - at about -93dB by the time you've taken dithering into account, but that is far too low a value to aim for with speech; you need to preserve a natural-sounding environment. Incidentally I don't think that meeting the ACX requirements will be affected too much by where the actual noise floor is - although I'm also slightly surprised that they've let your files through - the stipulation according to them is measure between -23dB and -18dB RMS and have -3dB peak values and a maximum -60dB noise floor and your noise floor is higher than that, at about -48dB, as I mentioned earlier. The Stephen Fry example looks like a good compromise to me, and should be the sort of silence level you should be looking for. At -48dB the noise will sound like a digital version of cassette tape hiss - which is pretty much what it is! And it's not helped by being an MP3 file either - just creating that will add a bit of noise - it always does. Which is why you need your file to have a SNR significantly lower before you convert it to the MP3 you're going to submit.

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