Micronas SC4

• Codec ID: 0x350
• Samples: http://samples.libav.org/A-codecs/Micronas%20SC4/

This is a simple speech codec supporting 32000, 16000, 11025 and 8000 Hz sampling rates.

It uses adaptive 6-point filtering and 30-byte blocks expanding into 58 samples (interleaved in case of stereo).

Codec data starts with a 5-byte header followed by 30-byte data frame and after every 100 frames there's a special 4-byte marker inserted depending on sample rate:

• 8000/11025 Hz -> 00 00 01 1B
• 16000/32000 Hz -> 00 00 02 2B

Each frame consists of 58 nibbles (starting from high nibble) and 0x55 sync marker at the end.

Nibble is expanded into sample in the following way:

 diff_sign = nibble >> 3;
 diff = STEP_TABLE[nibble];
 step = diff + (last_step >> 2);
 adiff = ((diff >> 11) == 0) ? ((newstep & 0x7F) + 0x80) << 7 >> (14 - (diff >> 7)) : 0;
 delta = diff_sign ? -adiff : adiff;
 new_sample = pred + delta;
 update weights as weight[i] * 255/256 - sign3(delta) * stored_sign[i] / 128
 shift weights back, assign delta as new weight[0]
 update stored signs
 calculate new prediction from the new_sample and dot product of weights and stored signs
 clip and output sample

Step table:

 0x800, 0x004, 0x087, 0x0D5, 0x111, 0x143, 0x175, 0x1A9,
 0x1A9, 0x175, 0x143, 0x111, 0x0D5, 0x087, 0x004, 0x800