AN1836-AN21161 Analog Devices, AN1836-AN21161 Datasheet - Page 7

• Manufacturer Part Number: AN1836-AN21161
• Description: Interfacing the ADSP-21161 SIMD SHARC DSP to the AD1836 (24-bit/96 kHz) Multichannel Codec
• Manufacturer: Analog Devices

seen by the AD1836's D/A converters and generate audible noise for many recursive algorithms with stringent filter
specifications.
The same audio processing algorithm implemented on the ADSP-21161 (which is the case when porting the same ANSI C code
from one processor to the other) would ensure these errors are not seen by the D/A converter. The ADSP-21161 32-bit DSP's
fixed-point support has 14 bits below the noise floor, allowing for the greatest SNR computation flexibility in developing
stable, noise free audio algorithms using the AD1836.
Just as we made assumptions about human hearing, for years it was taught that 24-bit signal processing generally offers
adequate precision for 16-bit samples. With higher-precision 24-bit converters emerging, what processor word width will we
recommend to maintain 24-bit precision? A 24 bit DSP might no longer be able to adequately process 24-bit samples without
resorting to double-precision math, especially for recursive 2nd-order IIR algorithms.
-124dB
-172dB
-72dB
-85dB
+20dB
Figure 5. Fixed-Point DSP Noise Floor Comparison AD1836's
0 dB
ADC/DAC Noise
AD1836 24-bit
“Floor”
The higher the number of bits, the more flexibility for round-off and truncation
errors as long as errors to not exceed noise floor of the A/D and D/A Converters.
Dynamic
with the 24-bit ADC/DACs's 105 dB Noise Floor
Range
96 dB
16-bit DSP
Dynamic
144 dB
Range
6 extra ‘footroom’ bits
below noise floor
24-bit DSP
Headroom
20 dB
Dynamic
24-Bit D/A Sample
192 dB
Range
Figure 6. 24-bit AD1836 ADC Samples vs DSP Dataword Sizes
24-Bit A/D Sample
16-Bit Data Word
31 30 29 28 27 26 25 24 23 22 21 2019 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4
23 22 21 2019 18 17 16 15 14 13 12 1110 9 8 7 6 5 4 3 2 1 0
1514 13 12 11 10 9 8 7 6 5 4 3 2 1 0
23 22 21 2019 18 17 16 15 14 13 12 1110 9 8 7 6 5 4 3 2 1 0
23 22 21 2019 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
24-Bit Data Word
32-Bit Data Word
largest ‘guard-band’ for
14 extra ‘footroom’ bits
below noise floor gives
compute errors
32-bit DSP
Processing Routines
Data Transfer of a 24-bit Audio
Execute DSP
Sample to an N-bit DSP for
Processing
D/A SNR
A/D and
105 dB
Computation Errors Must Remain
AD1836 ADC Noise Floor
16-bit processing not
To Maintain 24-Bit Precision,
possible to preserve
24-bit signal quality
Below Noise Floor
3 2 1 0