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Topic: Bitrate limit with QAAC and other encoders (Read 934 times) previous topic - next topic
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Bitrate limit with QAAC and other encoders

Hi.
I just learned that Apple AAC (QAAC) had a limit of available bitrate according to sample rate and channels (e.g. for 32KHz mono you can't go over 96kbps).

So I want to know if that's only true for QAAC, or if other AAC encoders (FDK, Nero) have the same limit, and more generaly other formats (MP3, OGG, AC3...).

If yes is there a quick way to know all of those limits (available bitrates) for the most common existing format (Google doesn't understand what I want)

Thank you.

Re: Bitrate limit with QAAC and other encoders

Reply #1
Yes, it's true. Confirmed here, the Apple encoder will cap 32KHz to 96 kbps per channel. (I tested stereo too.) It seems low, considering the encoder cap for 44KHz is 256 mono. (32 isn't that far from 44.)

A cap is necessary due to the limits of the iPhone AAC decoder. But even then, we're talking something 8 times the sample rate per channel in kilobits. (At 44KHz, a single block over 352k per channel will skip playback of a track.)

Testing other encoders at 32KHz, I've found:
  • Nero appears to peak at 182k/channel.
  • FDK seems to have a 128k/channel limit before padding in CBR mode. Note that it won't go nearly that high with VBR-5.
  • FhG/WinAmp will resample to 64kHz (weird), and refuse to encode higher than 288k/channel.
  • FAAC can go over 285k/channel.

Other formats don't have the same caps. For 32KHz, MP3 has the same 320k total limit for both mono and stereo. (Not counting free format.) Newer formats like Vorbis & Opus appear to be 256k/channel maximum regardless of sample rate, and are encoding over 200k/channel. (Even 8KHz, where 504k stereo is overkill.)

Re: Bitrate limit with QAAC and other encoders

Reply #2
Yes, it's true. Confirmed here, the Apple encoder will cap 32KHz to 96 kbps per channel. (I tested stereo too.) It seems low, considering the encoder cap for 44KHz is 256 mono. (32 isn't that far from 44.)

A cap is necessary due to the limits of the iPhone AAC decoder. But even then, we're talking something 8 times the sample rate per channel in kilobits. (At 44KHz, a single block over 352k per channel will skip playback of a track.)

Testing other encoders at 32KHz, I've found:
  • Nero appears to peak at 182k/channel.
  • FDK seems to have a 128k/channel limit before padding in CBR mode. Note that it won't go nearly that high with VBR-5.
  • FhG/WinAmp will resample to 64kHz (weird), and refuse to encode higher than 288k/channel.
  • FAAC can go over 285k/channel.

Other formats don't have the same caps. For 32KHz, MP3 has the same 320k total limit for both mono and stereo. (Not counting free format.) Newer formats like Vorbis & Opus appear to be 256k/channel maximum regardless of sample rate, and are encoding over 200k/channel. (Even 8KHz, where 504k stereo is overkill.)

Ok wow. Thanks for your tests.

At least FDK limit to 128kbps for 32KHz mono which is very very decent.

But i'm kinda disappointed by QAAC, which is known to be the best AAC encoder but have such limitations... I wanted to store old recordings (low audio frequency) at 32KHz but I won't after seeing that.

 
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