AD1954 Analog Devices, AD1954 Datasheet - Page 12

• Manufacturer Part Number: AD1954
• Description: SigmaDSP Digital Audio Processor
• Manufacturer: Analog Devices

AD1954
SIGNAL PROCESSING
Signal Processing Overview
Figure 2 shows the signal processing flow-graph of the AD1954.
The AD1954 is designed to provide all common signal-processing
functions commonly used in 2.0 or 2.1 playback systems. A
seven-biquad equalizer operates on the stereo input signal. The
output of this equalizer is fed to a two-biquad crossover filter
for the main channels, and the mono sum of the left and right
equalizer outputs is fed to a three-biquad crossover filter for
the SUB channel. Each of the three channels has independent
delay compensation. There are two high-quality compressor/
limiters available: one operating on the left/right outputs and one
operating on the subwoofer channel. The subwoofer output
may be blended back into the left/right outputs for 2.0 playback
systems. In this configuration, the two independent compressor/
limiters provide two-band compression, which significantly
improves the sound quality of compressed audio. In addition, the
main channels have a stereo “widening” algorithm that increases
the perceived spread of the stereo image.
Most of the signal processing functions are coded using full 48-bit
double-precision arithmetic. The input wordlength is 24 bits,
with two extra “headroom” bits added in the processor to allow
internal gains up to 12 dB without clipping (additional gains can
be accommodated by scaling down the input signal in the first
biquad filter section).
Each section of this flow-graph will be explained in detail on the
following pages.
Numeric Formats
It is common in DSP systems to use a standardized method of
specifying numeric formats. To better comprehend issues relating
to precision and overflow, it is helpful to think in terms of frac-
tional two’s complement number systems. Fractional number
systems are specified by an A.B format, where A is the number
of bits to the left of the decimal point, and B is the number of
bits to the right of the decimal point. In a two’s complement
system, there is also an implied offset of one-half of the binary
range; for example, in a two’s complement 1.23 system the legal
signal range is –1.0 to (+1.0 – 1 LSB).
RIGHT
LEFT
IN
IN
DEEMPH
DEEMPH
HPF/
HPF/
SUB CHANNEL
L/R MIX
EQ AND CROSSOVER
7 BIQUAD
7 BIQUAD
FILTERS
FILTERS
CROSSOVER
(3 FILTERS)
PRELIMINARY TECHNICAL DATA
CROSSOVER
CROSSOVER
(2 FILTERS)
(2 FILTERS)
SUB DYNAMICS PROCESSOR
1 BIQUAD
FILTER
(0–3.7ms)
(0–3.7ms)
DELAY
DELAY
LEVEL DETECT,
LOOKUP TABLE
L/R DYNAMICS PROCESSOR
The AD1954 uses two different numeric formats; one for the
coefficient values (stored in the parameter RAM) and one for
the signal data values. The coefficient format is as follows:
Coefficient Format
Coefficient format: 2.20
Range: –2.0 to +(2.0 – 1 LSB)
Examples:
1000000000000000000000 = –2.0
1100000000000000000000 = –1.0
1111111111111111111111 = (1 LSB below 0.0)
0000000000000000000000 = 0.0
0100000000000000000000 = 1.0
0111111111111111111111 = (2.0 – 1 LSB)
This format is used because standard biquad filters require
coefficients that range between +2.0 and –2.0. It also allows
gain to be inserted at various places in the signal path.
Internal DSP Signal Data Format
Input data format: 1.23
This is sign-extended when written to the data memory of the
AD1954.
Internal DSP signal data format: 3.23
Range: –4.0 to +(4.0 – 1 LSB)
Examples:
10000000000000000000000000 = –4.0
11000000000000000000000000 = –2.0
11100000000000000000000000 = –1.0
11111111111111111111111111 = (1 LSB below 0.0)
00000000000000000000000000 = 0.0
00100000000000000000000000 = 1.0
01000000000000000000000000 = 2.0
01111111111111111111111111 = (4.0 – 1 LSB).
(0–2.3ms)
(0–2.3ms)
DELAY
DELAY
LEVEL DETECT,
LOOKUP TABLE
(0–3.7ms)
DELAY
MONO DAC
L/R REINJECTION
INTERPOLATION
INTERPOLATION
LEVEL
SUBWOOFER
8
8
LEVEL
DAC
DAC
RIGHT
LEFT
OUT
OUT