.WV File Format – Entropy coding

Medians (ID_ENTROPY_VARS)

Three 16-bit little-endian values per channel, so 6 bytes for mono and 12 for stereo. Each is in the logarithmic form expanded by wp_exp2s.

GET_MED(n)  = (median[n] >> 4) + 1

inc(n):  median[n] += ((median[n] + DIVn) ÷ DIVn) × 5
dec(n):  median[n] −= ((median[n] + DIVn − 2) ÷ DIVn) × 2

DIV0 = 128, DIV1 = 64, DIV2 = 32

Reading a value

1. Count the 1 bits up to a 0: this is the unary count.
2. If the count reaches 16 (LIMIT_ONES), an escape follows: an Elias-gamma number added to 16. A larger count than that is an error.
3. Split the result into a parity bit and a count, using the pending holding_one and holding_zero state left by the previous value. The parity bit becomes holding_one, and half the value (plus 1, if the last holding_one was set) is ones_count. An even value sets holding_zero, so that the next value has a ones_count of 0 with no bits read.
4. Choose the range from the medians:

Then read the value within the range, and read one sign bit: 1 means negative, and the sample is ~value. Otherwise it is value.

The truncated binary code

To read a number up to max: a max of 0 reads nothing, and 1 reads one bit. Otherwise let bits be the bit length of max, and extras = 2^bits − max − 1. Read bits − 1 bits. If the value is under extras, that is the number. If not, read one more bit, and the number is (value × 2) − extras + bit.

Runs of zeros

When both channels’ median[0] are 0 or 1 and nothing is pending, a run of zeros may be coded. An Elias-gamma number gives the run length: count the 1 bits to a 0 (cbits), which is the length itself if under 2; otherwise read cbits − 1 more bits under an implicit leading 1. A nonzero run clears both channels’ medians. Each sample in the run is then decoded as 0 with no bits read. A run of length 0 falls through to normal decoding.

Hybrid mode (ID_HYBRID_PROFILE)

The payload holds these, one value per channel each, in the order:

Each block’s error limit is recalculated from these, using (slow_level + 128) >> 8. When the limit is not 0, the range is narrowed bit by bit until it is no wider than the limit, instead of a truncated binary read. In a hybrid file with a correction stream, a truncated binary read of the WVC stream then gives the exact value.

Noise shaping (ID_SHAPING_WEIGHTS)