I wanted to know how to convert the M4A/AAC created by get_iplayer after
downloading DASHhigh radio. I was going to ask this as a question but I
have now partially answered it myself. I thought I would post it here in
case it is of use to anyone else. Since I started looking at it Budge has
asked about creating a preset. I'll leave others more knowledgeable than I
to answer his question.
The first parameter I looked at was compression. The parameter
is -compression_level and it can be between 1 and 8, where 1 is the least
compression. For compatibility it is not recommended to go beyond 5. A
compression level of 8 does not increase the compression much, but I found
it halved the speed of conversion compared with a level of 5.
The command I used was
ffmpeg -i <infile>.m4a -f flac -compression_level 5 <outfile>.flac
I found that by default ffmpeg was creating files with 24 bit encoding,
which I did not want. The parameter for 16 bit encoding is
-sample_fmt s16
I found that ffmpeg created files with identical sizes if I
specified -compression_level 5 and if I did not specify
a -compression_level. It may be that 5 is the default, but curiously
specifying -compression_level 5 decreased the conversion speed by 12% with
24 bit encoding and increased it by 7% for 16 bit encoding. I decided to
use the default compression level.
My command then became
ffmpeg -i <infile>.m4a -f flac -sample_fmt s16 <outfile>.flac
The conversion took about 1% of real time which is faster than converting to
MP3 (7% of realtime at 128 kbit/s). The file size of an opera from Radio 3
downloaded as DASHhigh (320 kbit/s) increased by 54%. For speech the
increase in size is greater. The News Quiz Extra programme mentioned by
Budge was downloaded at 320 kbit/s and it doubled in size when converted to
FLAC. The increase would have been even greater if it had been converted
from 128 kbit/s.
While searching I found this
http://lists.infradead.org/pipermail/get_iplayer/2015-September/008221.html
which I think was a response to a question I had asked. My belated thanks
to Jim for suggesting FLAC as an option.
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