Dither to Mp3?

James Hardiman

Hi Bob,

It’s been a while, I hope you are doing well. Dare I say — I miss our loudness group’s regular calls!

I’m writing to ask for your advice. My colleagues at Vox and I are having a discussion about encoding. All of our podcast content is distributed at 192kbps stereo MP3. The question on the table is — when encoding 24-bit WAV to MP3, is dither necessary?

From my understanding, bit-depth only applies to PCM. Pro Tools (and some other DAWs) won’t even give you the option of selecting a bit-depth when exporting to MP3. So does that negate the need for dither?

I found one source that say that some older software or devices might actually convert the MP3 to 16-bit PCM on playback. In that case, dither is helpful. Otherwise, I’m thinking it’s not.

Might you have any advice to offer on this?

Best,

Rob

Hi, Rob:

No need to dither to mp3. The encoder is 32 bit float. It will try to fit as much of a 24 or 32 bit source into the mp3 encode as possible. Since it’s a coded system, the output of the 16 bit encoded word will look like 24 bits, with about 18 of them psychoacoustically useful, but use them all and treat them like 24 bits in the DAW. 

As far as older playback devices or codecs that apparntly convert mp3 or AAC to 16 bit on playback, your best safeguard is to inspect the output of any encoder with a free plugin from Stillwell audio called: Bitter. Try it… it’s very helpful. if it comes out as 16 bits, get rid of that codec. There is also some confusion since the codec packs the coded information into a 16 bit word, but its effective psychoacoustic resolution is around 18 bits or possibly more when it is reproduced, and Bitter will probably show 24 bits active on the output of the codec, which is normal and expected behavior.

How’s that sound!

Bob

Bob,

Thank you for this. You have soundly settled our debate 🙂

I will check out the Bitter plug. We have two MP3 encoders in use by our engineers — Fraunhoffer and LAME. I hope they both pass the test.

I and the team here appreciate your guidance on this!

Best,

Rob