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

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

A/D-D/A Convertors

Lab Signal Flow Mic Level Via USB

Tracking Studio Signal Flow Line Level Mic Level Via Firewire

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

Old Cart Signal Flow Via Lightpipe Mic Level Via Firewire

New Cart Signal Flow Mic Level Via USB

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

Bit Depth Sample Rate https://documentation.apple.com/en/soundtrackpro/usermanual/index.html#chapter=B%26section=2%26tasks=true http://thehub.musiciansfriend.com/tech-tips/sample-rate-and-bit-depth-an-introduction-to-sampling The process of A/D conversion is quantization, and when we quantize, there are errors which we call quantization or rounding errors.

https://documentation. apple. com/en/soundtrackpro/usermanual/index https://documentation.apple.com/en/soundtrackpro/usermanual/index.html#chapter=B%26section=2%26tasks=true

https://documentation. apple. com/en/soundtrackpro/usermanual/index https://documentation.apple.com/en/soundtrackpro/usermanual/index.html#chapter=B%26section=2%26tasks=true

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

http://www.ou.edu/class/digitalmedia/articles/ColorPalettes_Dithering_BitDepth.html

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

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

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

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

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

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

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

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

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

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