[FFmpeg-user] mono or stereo? ffmpeg converts or phone?

[Sean Darcy]

On 04/28/2014 05:53 PM, Sean Darcy wrote:
> On 04/28/2014 03:04 AM, Carl Eugen Hoyos wrote:
>> Sean Darcy <seandarcy2 <at> gmail.com> writes:
>>
>>> So for a bitrate of let's say 48000, am I better off
>>> converting to mono in ffmpeg ( -ac 1 ) or leaving it
>>> stereo and letting the phone convert it?
>>
>> You may be better off (and if playback is only mono, you
>> don't loose anything) but please understand that you cannot
>> double the quality / half the bitrate.
>> The chosen encoder is much more important, if your phone
>> does not support opus, make sure to choose libfdk-aac.
>>
>> Carl Eugen
>>
>
> I don't have an audio book reader that can read opus installed on my
> phone. I don't even know if there is one. I do use opus for voip: it's
> really amazing.
>
> But back to the OP. I am using libfdk-aac. I understand that 24000 mono
> will not equal 48000 stereo.
>
> My real question is: how does ffmpeg downmix 2 -> 1. I assume it doesn't
> just take one channel. Does it "average" in some way the two channels?
>
> I also assume whatever ffmpeg does, it does it better the android code
> on a phone doing it in realtime.
>
> sean

More specifically. My concern is file size on the phone. It seems to me 
I'll always have smaller file sizes for the same quality (in mono) if I 
convert to mono first. Maybe not double the quality / half the bitrate, 
but for any given bitrate, half the size. If I have a 48000 bitrate with 
two channels, isn't that file size ~twice the file size of a mono 48000 
bitrate?

sean