On 23/01/17 23:24, RS wrote:
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.
Hi Richard,
I think you have hit the nail on the head here. The AAC download from
GiP is not lossless but is the best resolution available. If I convert
to FLAC the file size increases substantially for no benefit. (I have
say 450 m4a files all over 300 MiB so we are looking at a big bite out
of my storage capacity if I convert.
My understanding is that if I convert to mp3 I lose more. Not sure if
this is true. Also mp3 is "proprietary." What I would like is just to
have the m4a downloads work once more. Now this applies to my main
collection and I would like to put that problem aside and forget the
Linn large file issue for now.
I am now talking about current half hour radio program downloads which
suddenly no longer play.
Nobody else has reported the recent problem with relatively short files
so I do not know if this is a bug in GiP or my system and nobody else is
joining this discussion.
Where next? Perhaps a new thread.
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