U. Glaeser

y a psychoacoustic audibility model, in order to meet the bit rate and the global threshold of hearing
requirements. The whole procedure is described below.
The psychoacoustic model filter bank is based on a 512-point FFT for layer I and on a 1024-point
FFT for layers II and III. First, samples of the spectral power density P(k) and the respective level L P(k)
in decibels are computed:
© 2002 by CRC Press LLC
P( k)
X( k)
2
=
LP( k)
= 10 log P( k)
(27.61a)
(27.61b)
where X(k) are DFT spectrum samples defined by Eq. (27.39a). Next in each of i = 0, 1,…,31 subbands
the signal SPL is computed as
Lpi = max[ LPmax i, 20 log10( 32768ssf max i)
– 10]
(27.62)
where L Pmax i is the maximum L P(k) value in ith subband.
Next, the relevant masker levels L Pm(z j) are searched for: tone masker levels L Ptm(z j), j = 1, 2,…,m tm,
and noise masker levels L Pnm(z j), j = 1, 2,…,m nm. Then, the masking indices are computed (in dB):
atm( zj) = – 1.525 – 0.275zj – 4.5
anm( zj) = – 1.525 – 0.175zj – 0.5
where by z j the jth critical band index in Bark is denoted.
Individual masking threshold levels are computed (in dB) as
Lttm( z, zj, LPm) = LPtm( zj) + atm( zj) + v( ∆z, zj) Ltnm( z, zj, LPm) = LPnm( zj) + anm( zj) + v( ∆z, zj) (27.63a)
(27.63b)
(27.64a)
(27.64b)
for tone maskers and for noise maskers, respectively. The so-called masking function v(∆z, z j) is defined by
v( ∆z, zj) =
⎧ 17( ∆z + 1)
– 0.4L Pm( zj) + 6
⎪
⎪ ( 0.4L Pm( zj) + 6)∆z
⎨
⎪
– 17∆z
⎪
⎩
( ∆z – 1)
( – 17 + 0.15LPm ( zj) ) + 17
(27.65)
where ∆z = z − z j. Expression (27.65) gives significant values in range −3 ≤ ∆z ≤ 8 only. Outside this
region we can assume that v(∆z, z j) → −∞.
Using expression (27.22), the global threshold of hearing L ptg (Fig. 27.14) can now be computed (in dB):
where L ptq(z) is the threshold of audibility in quiet. Consequently,
m tm
(27.66a)
(27.66b)
for ith subband is computed over all critical bands j contained in this subband. Now in each subband
the signal-to-mask ratio SMR i can be computed (in dB):
10
for
for
for
for
∆z < – 1
– 1 ≤ ∆z < 0
0 ≤ ∆z < 1
1 ≤ ∆z
Lptg( z)
10 log10 10 Lptq z ( )/10
10 Lttm z, zj , L ( Pm)/10
∑
10
j=1
Ltnm z, zj , L ⎛ ( Pm)/10⎞
= ⎜ + + ∑
⎟
⎝ j=1
⎠
Lptg min() i = min( Lptg( zj) )
SMRi =
Lpi – Lptg min() i
m nm
(27.67)