Subject:SFP2 save as AAC doesn't use custom settings?

Posted by: DSandberg

Date:6/7/2014 2:34:05 PM

I am trying to save a 256K AAC file. This is not one of the default presets in SFP for the "Apple MPEG-4 Audio" format, but opening the Options dialog (in the File Save As dialog) allows me to select a bitrate of 256,000, and when I click on the Ok button of the Options dialog, the Preset box on the Save dialog changes to show "Custom: AAC 44,100 Hz, stereo, 256000 bps", which is as I would expect. However, the bug is that this custom setting apparently is ignored by the actual save processing ... the resulting file is always 128 Kbps, not 256.

Subject:RE: SFP2 save as AAC doesn't use custom settings?

Reply by: SonySQAPete

Date:6/9/2014 11:17:14 AM

This isn't the first time that this question has been raised.

Forge is adhering to the user-specified custom settings. The "bitrate" as displayed on an AAC file is a bit of a misnomer.

These aren't constant bitrate files - the encoder will discard an amount of data during the encoding process that'll scale with respect to the complexity of the given audio stream. Apple would refer to the bitrate number that we're returning as the "nominal" bitrate.

An easy test would be to synthesize a 400hz sine wave and save it as a 256K AAC - due to the simplicity of the waveform, the bitrate will actually be listed as 28K!

It'd probably be more helpful to consider the bitrate in the save-as dialog to be a ceiling of sorts. Setting it higher won't automatically result in a higher nominal bitrate. It'll only make use of the additional bitrate if the encoder decides that it's actually needed for a given stream of audio.

Subject:RE: SFP2 save as AAC doesn't use custom settings?

Reply by: DSandberg

Date:6/9/2014 2:00:00 PM

Hmm. I'm not an expert in AAC file formats ... all I know is that, when I bounce an AAC file directly out of Logic Pro X, I get a file that is twice the size and which Sound Forge Pro reports as having a bit rate of 255 kbps. When I try the same save (i.e., using the 24 bit lossless file I dumped out of Logic for that same piece of music) out of SFP the resulting file is smaller and, when loaded by SFP, is reported as having a bit rate of 128 kbps. Is Logic using a different encoder that is capable of creating higher quality files?

Subject:RE: SFP2 save as AAC doesn't use custom settings?

Reply by: SonySQAPete

Date:6/10/2014 4:07:35 PM

Logic is opting to render their m4a files out as constrained VBR files, whereas Forge is rendering them as nominal VBR files. (Nominal VBR was also called "True VBR" in prior years.)

The nominal files aim for the highest quality while minimizing file size. The constrained VBR files don't make any effort to minimize filesize.

You can duplicate these results on your own using the afconvert utility that ships with OS X. Specifying a bitrate of 256000 & a "strategy" of 3 (for VBR) will result in a file that's nearly identical to Forge's output.

Rendering the same file with afconvert, a bitrate of 256000 and a strategy of 2 (for constrained VBR) will result in a file much like the output of Logic.

Neither method is right or wrong, but I went ahead and added the ability to change the codec strategy as a feature request. I'll bring it up with the team and we'll discuss adding it for a future release.

Subject:RE: SFP2 save as AAC doesn't use custom settings?

Reply by: DSandberg

Date:6/10/2014 5:25:48 PM

I see. Great information, and thanks for submitting the feature request. Greater flexibility in file format saving would always be welcome.