Gaplessness....

[MarmiteSandwich]

On the subject of renderers which do/don't support gapless. I have recently changed my renderer from mpd on a Raspberry Pi (£50) to a Cambridge Audio CXN V2 (£700). In both cases I observed that for some albums where gaplessness is important, although the decoded output to the DAC is nearly gapless, a very frustrating micro-gap can be heard, specifically with aac files in .m4a format. I estimated it to be c60ms. The same albums/tracks when encoded with flac did not display this characteristic.

I managed to get Cambridge Audio to admit that it was an audible flaw in .m4a/aac decoding and they have agreed to solve it in a forthcoming firmware release. That was very impressive, I thought.

Meanwhile, I had tracked the problem, via the developer of mpd to a "bug" in ffmpeg, a piece of software which is widely used in audio encoding/decoding. I suspect that CA are one user of ffmpeg. I registered it with ffmpeg as a bug, and recently received a reply which rather surprised me. It turns out that .m4a/aac files created with iTunes are not supported by ffmpeg for gapless processing. Something to do with null samples in the data at the beginning and end of the track, and how to tell the decoder how many there are. iTunes uses a iTunSMPB tag in the metadata to do this and ffmpeg doesn't recognise this tag. More info which I don't understand here.

Since CA have said they will fix the problem, I am not overly concerned, but I would be interested if anyone can recommend a way of converting these iTunes files to something which ffmpeg will play truly gaplessly. Somebody at ffmpeg was very helpful about this, but I couldn't understand the response.
Marmite