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Matteo Cypriani 6 years ago
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# About the NRG image format
A good reference about the metadata format is the corresponding Wikipedia
article: <https://en.wikipedia.org/wiki/NRG_(file_format)>.
The source code and comments of NRGrip also provide some information, of
# Manual audio data extraction howto
Just for fun, it is possible to extract manually the audio data from an NRG
image, after displaying its metadata with NRGrip. You will need to know:
- The first audio byte (`dd skip`):
`daox.first_track.index1` (or `index0` if you wish to include the pre-gap)
- The last audio byte:
- The number of audio bytes (`dd count`):
`Last audio byte - First audio byte`
To extract the raw audio data (substitute `${FIRST_AUDIO_BYTE}` and
`${NB_AUDIO_BYTES}` with the numbers found above:
dd iflag=count_bytes,skip_bytes if=image.nrg of=image.raw \
bs=4M skip=${FIRST_AUDIO_BYTE} count=${NB_AUDIO_BYTES}
# Notes on cue sheet extraction
From Wikipedia:
- A CD can contain up to 99 tracks.
- Each track can in turn have up to 100 indexes.
- The vast majority of songs are recorded under index 1, with the pre-gap being
index 0.
- Sometimes hidden tracks are placed at the end of the last track of the disc,
often using index 2 or 3.
- In an NRG cue sheet, the _index0_ points are present even when they are
identical to the _index1_ ones.
- The _index0_ points in audio tracks are incorrect if Nero has been asked to
record all the sub-channel data (in that case the sector size is 2448 bytes).
- No index other than 0 or 1 has been encountered, although the chunk format
allows for such cue points to be recorded; thus the number of cue blocks seems
to always be `2 * (number of tracks + 1)`: two indices for each track, an
_index0_ for the lead-in and an _index1_ for the lead-out.
From this information and other observations:
- The cue sheet should include all the indices for each track, but _index0_
should be ignored if:
- _index0_ = _index1_, or
- _index0_'s position is negative.
- It is possible to have incoherent track numbering (observed: track 10 is
numbered 16 in CUEX).
- The lead-in area (track 0) and the lead-out area (track 0xAA) should not
appear in the cue sheet.
How will multi-session CDs be handled? Is it worth implementing, since
multi-session audio CDs must be very rare?