USB Audio AES Oslo 2015 04 13 Brge

USB Audio AES Oslo 2015 -04 -13 Børge Strand-Bergesen

Børge Strand-Bergesen • Has been making a CD player for >20 years • Experience from design and manufacturing of electronics • The CD player was in need of a USB input • The Open Source SDR Widget was better than commercial USB chips 5 years ago • Has commercialized the Audio Widget project • Has a daytime job as Product Manger at Miros AS 2

Introduction • • What is time? What is digital audio? What does a DA converter really do? How is digital audio transferred? Why is an asynchronous protocol so good? Tips and tricks for good digital audio Demo 3

What is time? • We notice how fast things happen – Changes in rhytm – Offset hole in LP – Timing in DA converter • Computers are not as sensitive – As long as things happed before they are due – The clock may well vary 4

What is digital audio? • Sound is an analog phenomenon – Continuous signals – No sudden changes – Noise = “Fireplace” • Digital sound has undergone a translation – “Chopped up” in time – Rounded off to nearest integer – Noise = Quantization & Jitter 5

What does a DAC do? • The DAC makes the audio signal continuous again – It plays back each sample – It fills in the gaps = + 6

Errors in DACs Digital audio has two fundamental errors: • Out of the DAC vs. into the ADC • A CD has the samples rounded off to the closest of 65536 available integers (16 bits) • More bits help! – Jitter, timing errors in when the samples are played back • Darn good clocks help • … and the right transfer method → Level (quantization) – Quantization noise, level errors → Time (jitter) 7

How is digital audio transferred? • Synchronous protocol – – Transmitter controls the timing Receiver regenerates clock signal SPDIF (coax) and TOSLINK (optical) Some people can tell cables apart by their sound! – They carry a very digital data signal – and a very analog timing reference – Transmitter, receiver and cable are never 100% matched – Some CD players transmit well – Some DACs receive well – PCs in general don’t transmit well 8

Digital audio with USB • USB is packet based (1 or 8 packets/ms) – CD audio with 44. 1 ksps is sent as 9 packets of 44 samples and one packet of 45 – It is hard to synchronize to this scheme! 9

Transfer methods in USB audio Synchronous, DAC locks to USB clock – PCs are really, really bad at making good analog clocks! Adaptive, DAC has an adjustable clock – Adjustable clocks are expensive and somewhat noisy – Analog control loop Asynchronous USB, DAC controls timing – The DAC can be made with a built-in, really good analog clock – Digital control Asynchronous sample rate conversion – The clock in the DAC is completely independent of PC’s clock – Every single sample is digitally modified 10

Asynchronous transfer = good! The DAC becomes the timing reference – – Good clock chips placed right by the DAC chip The DAC controls the source USB is a very good fit Asynchronous USB says “Speed up” or “Relax” back to PC/Mac/i. Pad 11

Sample rate conversion 1. 2. 3. 4. The task has 44. 1 points per foot of line Draw it up with curve ruler #4 On translucent paper: draw 48 points per foot Ask Junior to solve the task 12

Sample rate conversion 1. 2. 3. 4. 44. 1 points / foot = Sample rate of CD Curve ruler #4 = digital FIR filter 48 points / foot = Sample rate out Junior = DAC Windows and OS X do this without asking you! This was expensive at the time of DAT. Today it has no cost. 44. 1 in and 48 out means: • Each 147. input sample == each 160. output sample • Every other sample is calculated, and different from the original! 13

Playback tips and tricks Decide ambition level 1. Everyday use • • • 4. Über geek Plug’n’play MP 3 You. Tube 2. Informed use • • CD quality Tidal / Wimp-Hi. Fi 3. Hi-res • • Needs a bit more setup >48 ksps No “pling, you got mail” Dedicated music player SW 14

Informed use on Windows 15

Advanced use on Windows – – If CD quality is sufficient, use WASAPI protocol Download ASIO driver from www. henryaudio. com Use DAC in UAC 2 mode (USB Audio Class 2) Use an ASIO compliant player program • • – Use a virtual audio device • • – foobar 2000 with its ASIO plugin JRiver Media Center http: //www. jriver. com/ Hi-Fi CABLE & ASIO Bridge http: //vb-audio. pagesperso-orange. fr/Cable/ Read the Henry Audio Blog 16

Informed use on a Mac 17

Informed use on i. Pad / i. Phone – – – Use latest i. OS Purchase Apple’s "Lightning to USB Camera Adapter“ or "Apple i. Pad Camera Connection Kit". Purchase a USB hub with its own power supply Use DAC in UAC 2 mode Read the Henry Audio Blog 18

Driver summary • • UAC 1 UAC 2 USB Audio Class 1 = OK for CD quality USB Audio Class 2 = Hi-Res, 192 ksps ● ● Windows OS X Linux i. OS Built-in driver with resampling. Works with all software. Built-in driver with resampling. Edit mpd-config. Doesn’t work with UAC 1 in current firmware. No OS support No “Pling …” Very good with ASIO and dedicated players. Built-in driver with resampling. Edit mpd-config. Built-in driver without resampling! 19

Demo! 20