AN1836-AN21161 Analog Devices, AN1836-AN21161 Datasheet - Page 6

• 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

Figure 4. Fixed Point DSP Comparisons and Their Relationship To Converter Dynamic Range/SNR
Figure 5 shows how a larger DSP word size enables highest-quality signals on the AD1836 output DACs, when audio signals
are digitally processed from a AD1836 A/D and it's 105 dB dynamic range and SNR. Imagine in this case the 24-bit sample is
transferred from an A/D to the DSP's internal memory. Notice that the 24- and 32-bit processors provide adequate 'footroom
bits' below the noise floor to protect against quantization errors.
However, a 16-bit DSP does not have the native data-word width to accurately represent and process the 24-bit sample (unless
double-precision routines are used). This is graphically shown in Figures 5 and 6 with the noise floor of the AD1836 lower
than the noise floor of the 16-bit DSP. For a 24-bit DSP, there is little room for error in arithmetic computations. For complex
audio processing using single-precision arithmetic, the 24-bit DSP dataword size may not be adequate for precise processing of
24-bit samples because of truncation and roundoff errors… without the use of double precision math for computationally
intensive audio algorithms. Errors produced from the arithmetic computations with 24-bit single precision math could easily be
0 dB
Fixed-Point DSP Dynamic Range Comparisons
Dynamic
Range
96 dB
Quantization
16-bit DSP
Levels of
65536
SNR
A D RMS
/ (
Dynamic
144 dB
Range
Quantization
)
24-bit DSP
16,777,216
(
Levels of
DSP Noise
dB
“Floor”
Established by
the A/D and/or
D/A converter
) = = = = 6 02
dB
- n
+m
+0
DSP/Converter SNR and Dynamic Range
Converter
Dynamic
.
Range
n
Dynamic
192 dB
Range
+ .
1 76
4,294,967,296
Quantization
32-bit DSP
Levels of
‘System’
dB
DSP
SNR
Headroom
“Floor”
Noise
N-Bit DSP Dynamic
Dynamic
Fixed Point DSP Quantization Level Comparisons
Quantization
Range & SNR
= 6 x N-bits
16-bit
Level
1
1 16-Bit Quantization Step = 256 24-bit DSP Quantization Steps
1 16-Bit Quantization Step = 65,536 32-bit DSP Quantization Steps
1 24-Bit Quantization Step = 256 32-bit DSP Quantization Steps
DSP Quantization
Error = +/- 1/2 LSB
Range
Headroom
Quantization
Levels of
(
24-bit
dB
256
)
=
Electronic
Line Level
Level
Conversion
Peak
Normal
. 6
Error
A/D
02
Quantization
n
16-bit
Level
+
1
. 1
76
dB
≅
6
Quantization
n
Levels of
65,536
32-bit