The decoder reverses the encoder’s steps. The received bitstream is unpacked to restore the spectral coefficients, then the SNS and TNS shaping is undone, and the spectrum is inverse-transformed back to the time domain. Finally, the LTPF postfilter is applied.

When BFI = 1 (bad frame), the decoder skips bitstream decoding entirely and invokes Packet Loss Concealment (PLC) to generate a substitute output frame.

Bitstream decoding mirrors bitstream encoding exactly. The decoder reads backward from byte nbytes−1 to get side information, and forward from byte 0 to run the arithmetic decoder. BEC (Bit Error Condition) checks are performed at multiple points — if BEC is detected, BEC_detect = 1 and the decoder stops parsing and may apply PLC.

bp       = 0;
bp_side  = nbytes − 1;
mask_side = 1;
c        = 0;
BEC_detect = 0;
rateFlag = (nbits > 160 + fsind*160) ? 512 : 0;

Side information is read in the exact same order it was written. BEC checks are performed for values that must be in known ranges:

The decoded Pbw value is mapped back to a bandwidth type (NB/WB/SSWB/SWB/FB) using Table 3.6. This bandwidth information controls the TNS filter range (Section 3.4.6) and the noise filling stop frequency bw_stop (Section 3.4.4).

The arithmetic decoder reads forward from byte 0. It is initialized with the first 3 bytes of the payload, then decodes symbols using the same probability tables as the encoder.

TNS coefficients: For each active TNS filter, rcorder(f) and rci(k, f) are decoded first.

Spectral data: For each 2-tuple k = 0, 2, 4, …, lastnz−2:

• Decode escape symbols until sym < 16 (max 14 levels — BEC if lev reaches 14)
• Read LSB bits from backward stream for each escape level
• Decode MSB symbol sym = |a| + 4×|b| from arithmetic stream
• Read sign bits from backward stream for non-zero a, b
• Accumulate magnitude from escape and MSB levels

BEC is also set if the forward pointer bp crosses the backward pointer bp_side by more than 3 bytes during spectral decoding.

After spectral decoding, coefficients beyond lastnz are set to zero. Then residual bits are read (in lsbMode=0) or LSBs are applied (in lsbMode=1) to refine the decoded spectrum.

Noise Filling Seed (nf_seed): A deterministic seed for the noise-filling pseudo-random generator is derived from the decoded spectrum:

Zero Frame Flag: If the decoded spectrum appears to be all zeros (PLC trigger condition):

When zeroFrameFlag=1, noise filling is skipped (Section 3.4.4). This prevents noise from being injected into intentionally silent frames.

/* read_bit: reads 1 bit from backward stream (mirrors write_bit_backward) */
int read_bit(bytes[], *bp, *mask) {
    bit = (bytes[*bp] & *mask) ? 1 : 0;
    if (*mask == 0x80) { *mask = 1; *bp -= 1; }
    else *mask <<= 1;
    return bit;
}

/* read_uint: reads numbits bits LSB first from backward stream */
int read_uint(bytes[], *bp, *mask, numbits) {
    value = read_bit(bytes, bp, mask);
    for i = 1 to numbits−1:
        value += read_bit(bytes, bp, mask) << i;
    return value;
}

/* ac_dec_init: init arithmetic decoder, consuming first 3 payload bytes */
void ac_dec_init(bytes[], *bp, *st) {
    st->low = 0;  st->range = 0x00ffffff;
    for i = 0 to 2: { st->low = (st->low << 8) + bytes[(*bp)++]; }
}

/* ac_decode: decode one symbol using cumulative frequency table */
int ac_decode(bytes[], *bp, *st, cum_freq[], sym_freq[], numsym, *BEC) {
    tmp = st->range >> 10;
    if (st->low >= tmp << 10) *BEC = 1;
    val = numsym − 1;
    while (st->low < tmp * cum_freq[val]) val--;
    st->low -= tmp * cum_freq[val];
    st->range = tmp * sym_freq[val];
    while (st->range < 0x10000):
        st->low = ((st->low << 8) & 0xffffff) + bytes[(*bp)++];
        st->range <<= 8;
    return val;
}