[FFmpeg-user] MP4s (and M4As) for music

[Ferdi Scholten]

On 08-06-2025 18:59, Mark Filipak wrote:
> On 08/06/2025 04.47, Ferdi Scholten wrote:
>> On 08-06-2025 04:21, Mark Filipak wrote:
>>> The script above provoked this: "Too many bits 15269.442177 > 12288 
>>> per frame requested, clamping to max". Of course, I have no idea 
>>> what that's about. Do you? "Too many bits"?
>> Simple, the fixed bitrate you specified is very much too high for the 
>> audio you provide. ...
>
> It is? I'm providing 657.6 k-bits/second/channel (i.e. pcm_s16le) and 
> I'm specifying '-b:a 657.6k' It seems right to me
>
> (41.1 k-samples/sec/channel)*(16 bits/sample)=657.6 k-bits/second/channel
>
> If that's not correct, then what bit rate does the encoder want? and 
> why would it want anything other than pcm_s16le? And if pcm_s16le at 
> 657.6 k-bits/second/channel is too fast, do I need to resample from 
> 41.1 k-samples/sec/channel to something lower? What do you suggest and 
> why?
The encoder wants the desired bitrate for the encode, not the incoming 
bitrate. As aac is a lossy codec it will compress your incoming stream 
and drop anything that does not fit within the profile. 128kbps is very 
common and used for most stereo audio on DVD's for example. 192kbps is 
common for 5.1 audio etc...
>
> I'm truly not happy with FFmpeg's lack of support for pcm-in-mp4.
>
>> ... The encoder is complaining about that. Either specify a lower 
>> bitrate or use variable bitrate instead.
>
> Thanks, Ferdi. Can you be more specific? Can you provide numbers, 
> bounds (i.e. upper and lower limits on the numbers), and units (i.e. 
> dimensions of the numbers)?
>
> The documentation is just too sparse and too illogical.
>
> b    Set bit rate   ...of what? the encoder's input? the encoder's 
> output?
>  in bits/s   ...total bits/s? bits/s/channel?
>  Setting this automatically activates constant bit rate (CBR) mode   
> ...fine, that's what I want
>  If this option is unspecified it is set to 128kbps   ...huh? if it's 
> unspecified, it's not cbr, is it? or is this telling me it's always cbr?
>
> q    Set quality   ...what are the units?
>  for variable bit rate (VBR) mode   ...how do you specify vbr?
>  This option is valid only using the ffmpeg command-line tool. For 
> library interface users, use global_quality   ...huh?
>
> There's not enough documentation there to know anything.
>
That is why usually the FDK-AAC encoder is used via libfdk_aac
It is better documented and offers more and better options for encoding 
to specific needs. Only drawback, it's not "free software"