1. Digital Audio Processing
Tufts University – ES 65 October 12, 2017

2. Signal Flow Mic Level Preamp Line Level A/D Convertor
5-50 mV
Preamp
Line
Level
50 mV – 2V
A/D Convertor
Digital Processing

3. A/D-D/A Convertors

4. Lab Signal Flow
Mic Level
Via USB

5. Tracking Studio Signal Flow
Line Level
Mic Level
Via Firewire

6. Mixing Studio Signal Flow
Via Firewire
Converter Outputs
Converter Inputs
Console Inputs
Console Outputs

7. Old Cart Signal Flow
Via Lightpipe
Mic Level
Via Firewire

8. New Cart Signal Flow
Mic Level
Via USB

9. Analog to Digital Variables
Bit Depth
Sample Rate
Latency
Clock Source
Jitter
Other things that are important but not necessary for use to talk about yet:
-effective number of bits
-INL/DNL errors
-power dissapation

10. Bit Depth
Sample Rate
The process of A/D conversion is quantization, and when we quantize, there are errors which we call quantization or rounding errors.

11. https://documentation. apple. com/en/soundtrackpro/usermanual/index

12. https://documentation. apple. com/en/soundtrackpro/usermanual/index

13. 3-bits
That is actually three bits
000
001+
011+
010+
100-
101-
111-

15. What’s this sound like? Tristan Perich All the sample rates

16. Aliasing
Problem:
When the recorded spectrum exceeds available spectrum before Nyquist theorm
Solution:
Filtering before the signal is recorded

17. Jitter
Problem:
Misrepresentation of the analog signal on record (A/D) or playback (D/A)
Solution:
Solid clocking

18. Clock Source 1 Bit of timing information Master vs. Slave
Word Clock vs. Embedded
External Clocks

19. Dither Problem: Distortion as we reduce bit depth Solution:
Adding filtered/shaped noise
-Dither is used to prevent truncation distortion when converting from high sample rates to lower ones by adding noise.
-Noise shaping helps move the applied noise out of the audible spectrum so it’s less noticeable
-Sounds pretty much like analog tape hiss, but you really hear it most when converting to 8 bits or below

20. Reconstruction Problem: Digital signal has square steps Solution:
Anti-imaging/Reconstruction Filter

21. Oversampling Problem:
Sharp anti-aliasing filter ahead of A/D conversion introduces ringing
Solution:
Oversample at conversion,
then downsample after conversion
You’re not storing the oversampled signal, you’re storing the regular one

22. Latency Time it takes to convert the signal A/D & D/A
Humans (and especially musicians) can easily perceive differences <10 ms
Zero latency monitoring combines analog input with digital output

23. Best Practices Use dither when reducing bit depth SRC with care
If you hear spurious clicks, it’s probably a clocking issue
Set up your own studio to make clocking and digital connectivity simple
Use high bit rates & sample depths when possible, but don’t go overboard

24. Next Steps ADC manufacturers close by DSP specialists nearby
Analog Devices
DSP specialists nearby
Mathworks