Converting DASHhigh to FLAC with ffmpeg

[Budge]

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.