OBD:SNDD: Difference between revisions

:The bottom line is that, both for PC demo SNDDs and for Mac SNDDs, the "1" flag should always be set.

The compressed storage of .raw data is described in the following two sections.

===IMA4 ADPCM .raw data (Mac)===

''For an overview of the IMA ADPCM algorithm and IMA4 header (if interested), see [https://wiki.multimedia.cx/index.php/Apple_QuickTime_IMA_ADPCM HERE]. For an actual implementation example, see, e.g., [[wp:FFmpeg|FFmpeg]].''

:The '''channel count''' is specified in the 50-byte header if said header is enabled, otherwise it is inferred from [[OBD:OSBD/OSGr|OSGr]].

:As for '''compression''', PC retail actually has ''three'' primary compression modes, commanded by the flag values. Only the first is used in Vanilla Oni.

===WAVE-like format header ("8" flag)===

If the "8" flag of the SNDD (at 0x08) is ON and the "4" flag is OFF (as is always the case in Vanilla Oni), the 50-byte block is interpreted as a standard "fmt " chunk that you find in WAVE files (see [[/wav|HERE]] for details).

:PCM playback is only known to work in PC retail builds, either with IMA4 and WAVEfmt headers disabled (the stream is simply copied to the output buffer, as Little-Endian 16-bit linear PCM, with the channel count specified at OSGr level) or with the WAVEfmt header enabled (in which case the stream can have a custom bit depth and channel count).

:In the case of PC demo, the PCM playback fails (the engine identifies the stream as already uncompressed and skips the initialization of ACM headers, but then proceeds with decompression anyway and stops because of zero input size). This causes no problems for impulse sounds (other than silence), but results in a crash for looping permutations (the same playback keeps failing over and over).

==Known data issues==

;PC SNDD data

The Mac version and the PC demo version use a simpler format, with no support for different sample rates (all sounds are sampled at 22050 Hz).

====.raw part (MS ADPCM, PC demo)====

For PC demo the .raw SNDD data is actually the same as for PC retail, but with the same short .dat header as on Mac. The ADPCM block size is 512 bytes for mono, and 1024 for stereo. The sample rate is 22.05 kHz.

====.raw part (IMA4 ADPCM, Mac)====

For an overview of the IMA ADPCM algorithm and IMA4 header (if interested), see [https://wiki.multimedia.cx/index.php/Apple_QuickTime_IMA_ADPCM HERE]

Also, from a careful examination of the sound stream that is actually played back by Oni in the main menu, it is clear that all the Oni engines (both Mac and PC) play back all the available data (including the "padding" of the fixed-size IMA4 blocks) before switching to the next segment. The frame count (number of game ticks) does not affect a sound's playback and is used only as an indication for subsequent sounds (e.g., for approximate cueing in [[BSL]]).

;NOTE

==Exporting and importing tips==

To create a wav/aif file one needs to write a file header like below and then write the contents of the raw data part.

===WAV files (from PC retail/demo SNDDs)===

*Write "RIFF"

* last_block_samples = (last_block_size - 7*n_channels)*(8/bits_per_sample/n_channels) + 2;      '''// EXAMPLE: (86 - 7)*(8/4) + 2 = 160'''

* n_samples = n_whole_blocks*samples_per_block + last_block_samples;        '''// EXAMPLE: 20*1012 + 160 = 20400'''

==Looping issues==

As detailed above, ADPCM data is stored in blocks, but the actual sound data does not necessarily end exactly at the end of a block. This is true both for MS ADPCM (PC retail or demo) and IMA4 ADPCM (Mac), but is especially noticeable for the comparatively large blocks of MS ADPCM, where the padding can be as large as ~1010 samples, i.e., a ~46-millisecond silence in the case of 22.05 kHz (for IMA4, the biggest possible gap is 63 samples, or ~3 milliseconds).

===MS ADPCM===

Although the final block of a MS ADPCM SNDD file (PC retail) is stored in incomplete form (with only the actual samples and no padding), the standard decoding behavior when loading an ADPCM-compressed WAV (e.g., in a non-destructive audio program) is to assume full-sized blocks, with padding up to the end of the last block. Depending on the audio program, this can create a silence or some "bad data" at the end of the imported audio, which can be a problem if one wants to join SNDDs that are supposed to play seamlessly one after another (e.g., a musical or ambient sequence).  

Slight distorsions are sometimes observed near the ends of looping SNDDs (music and ambient tracks). These artifacts were likely caused by Bungie's audio tools, and can not be undone automatically. Barely noticeable, they can be healed by manually editing audio samples near the seams.

===IMA ADPCM===

As a workaround/solution, the correct sample count of a Mac SNDD can be looked up in a PC counterpart (always available, since we're only talking of music/ambients/sirens, which are neither localized nor sampled at 44.1 kHz), and then used to trim the .aif file in FFmpeg, while converting to .wav (either PCM or ADPCM). However, it's easier (and more reliable) to just grab a PC retail copy of Oni and extract the MS ADPCM sounds.

====Initial transient====

The values of those initial samples is not recoverable (unlike the padding at the end of SNDDs, which can be trimmed down). Therefore, if working with sound samples extracted from Oni, it is recommended to turn to a PC version's SNDDs.

==PCM export and PC demo detection==

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==Notes==

<references/>