Audio Scriptures: Audio quality

[unixplumber]

I know this is an old thread, but I was wondering if anyone knew if there was anywhere to download the BOM audio in a lossless format. There's a newer audio codec called opus that does really well at compressing spoken word at low bitrates, much lower than mp3 or other lossy formats. I'd like to see how much I can compress it with Opus while still having acceptable audio quality, but I need uncompressed or high quality input files.

(Five years later....)

I also have recently looked for a lossless copy of the BOM voice files for the same reason, which led me to this thread. I couldn't find them either, so I just used the 64k MP3 files (the 2015-11 female voice version) to get a good idea of how much I can compress them while maintaining acceptable quality.

Here's what I found:

• 20k: Even with good headphones I can't tell this apart from the original MP3. (Note: I didn't do a proper ABX test, so take this with a grain of salt.)
• 14k: Noticeable (but not annoying) artifacts.
• 12k: Sounds a little "muffled" (seems to be cut off above 10kHz). Sounds somewhere between AM radio and FM radio.
• 8k: Sounds more muffled (seems to be cut off above 5kHz). Sounds nearly like an old school telephone or AM radio with some artifacts.
• 5k: Sounds like 8k but has very noticeable and annoying artifacts (sounds "choppy" or "fluttery"). (I tested this just to see how low Opus can go.)

For the "best" audio quality, equivalent to the original 64k MP3 files, 20-24k Opus should be enough (the Opus documentation recommends 24k for mono audiobooks/podcasts, and Hydrogenaudio claims that 24k Opus can produce "Near transparent speech"). Of course, it's entirely possible that a 24-32k Opus file could sound better than 64k MP3, but I can't test this theory since I don't have the lossless sources.

I could personally listen to 14k Opus for long periods of time. The artifacts are really not bad like MP3 is at low bitrates (MP3 artifacts sound "tinkly" or "bubbly" while Opus artifacts generally sound a little "rough" or "noisy", and they tend to be somewhat harder to hear than MP3 artifacts).

8k Opus is listenable (like AM talk radio) and is useful for someone with limited storage space and/or slow/capped Internet (think third-world countries).

(Caveat: some of this is subjective and is based on a sample size of one (me). I used the latest version of libopus, 1.3, which has significant improvements in speech quality since previous versions.)

(Also, note that the cover art in each file adds about 6k to the bit rate on average, regardless of format. So a 64k MP3 file actually has a bit rate of about 70k, and a 20k Opus file actually has a bit rate of about 26k.)