L64005 MPEG-2 Audio/Video Decoder Technical Manual
7-5
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
and n-m as shown in example 2.
Registers 49 through 51 in Group 6 contain the values for the NCO
numerator (n) and the NCO denominator (n-m). The registers are 12 bits
wide and the sign bit for n-m is included in the hardware. Refer to the
The NCO denominator (n-m) value is stored as a negative two’s comple-
ment number. For example, the value -3119 would equate to negative
0xC2F. The value stored for (n-m) would therefore be 0x3D1 which is the
two’s complement of 0xC2F.
The BCLK has a constant phase only if
is an integer. A phase con-
stant BLCK is generally necessary when using an oversampling DAC.
For this reason, the L64005 allows for an external DAC input clock to be
supplied via the ACLK pin. This is called the ‘slave mode’. This clock is
still passed through the NCO as described previously to generate BCLK,
but BCLK is now phase constant.
If the 27MHz SYSCLK is used to generate the BCLK, non-integer ratios
cause the phase to be jittered around the desired frequency. This is
called the “master mode’ and may be satisfactory for low end applica-
tions using in-expensive broadband DACS that require only a BCLK
input. However, this may not be accruate enough for most applications.
So the slave mode is generally recomended, and that the ACLK input be
supplied from the L64008/108 Transport chip or from an oversampling
DAC.
Table 7.1
Typical Values for NCO
at 27 MHz
fd
Nb = 32
Nb = 48
Nb = 64
Sample Frequency (kHz)
n
n-m
n
n-m
n
n-m
16.0
128
-3247
192
-3183
256
-3119
22.05
98
-1777
49
-576
196
-1679
24.0
64
-1061
32
-343
128
-997
32.0
256
-3119
384
-2991
512
-2863
44.1
196
-1679
98
-527
392
-1483
48.0
128
-997
64
-311
256
-869
mn