richard wrote:
On Tue Apr 12 Simon Nash wrote:
dinkypumpkin wrote:
When mp4tags rewrites the MP4 metadata on save, the underlying MP4
library does the arithmetic to calculate the necessary values and
fill in the esds atom. From perusing the ffmpeg code, it looks like
the required logic could be added, but I'm not sure it should be. I
don't think zero is an incorrect value for those fields, plus it
would sort of violate the codec-based model of ffmpeg.
This makes sense, but it doesn't explain why ffmpeg sets the
maxBitrate field. Where does it get the value from?
In order to understand the analysis of the metadata, below are extracts
for the files tested with the Marantz player.
Bells1_noscript.m4a is the only file that doesn't play. The average
bitrate is zero. The average bitrate in all the other files is non
zero.
test1.mp4 does not play (as the Marantz does not recognise a .mp4 file),
but change the extension to .m4a and it plays OK.
Bells1_noscript.m4a is downloaded with get_iplayer. Bells1_script.m4a is
the same file fixed up with Jon's script.
Simon can explain what the other files are.
Bells1_noscript.m4a
type ftyp
majorBrand = M4A
type esds
decConfigDescr
bufferSizeDB = 0 (0x000000) <24 bits>
maxBitrate = 143325 (0x00022fdd)
avgBitrate = 0 (0x00000000)
Bells1_script.m4a
type esds
decConfigDescr
bufferSizeDB = 620 (0x00026c) <24 bits>
maxBitrate = 135328 (0x000210a0)
avgBitrate = 128002 (0x0001f402)
Bells1_noscript_bitrate.m4a
type esds
decConfigDescr
bufferSizeDB = 0 (0x000000) <24 bits>
maxBitrate = 143325 (0x00022fdd)
avgBitrate = 128002 (0x0001f402)
This is Bells1_noscript with the avgBitrate field patched using a
hex editor.
Bells1_noscript_bitrate_buffer.m4a
type esds
decConfigDescr
bufferSizeDB = 620 (0x00026c) <24 bits>
maxBitrate = 143325 (0x00022fdd)
avgBitrate = 128002 (0x0001f402)
This is Bells1_noscript with the avgBitrate and bufferSizeDB fields patched
using a hex editor.
test1.mp4
type ftyp
majorBrand = mp42
type esds
decConfigDescr
bufferSizeDB = 1164 (0x00048c) <24 bits>
maxBitrate = 329224 (0x00050608)
avgBitrate = 320110 (0x0004e26e)
This is an ADTS .aac file converted to mp4 format using mp4creator
with -optimize.
test1a.m4a
type ftyp
majorBrand = M4A
type esds
decConfigDescr
bufferSizeDB = 1164 (0x00048c) <24 bits>
maxBitrate = 329224 (0x00050608)
avgBitrate = 320110 (0x0004e26e)
This is an ADTS .aac file converted to mp4 format using mp4creator
with -optimize and then patched using a hex editor to change the
ftyp atom major brand from mp42 to M4A.
Simon
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