USB Audio AES Oslo Børge Strand-Bergesen

2 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

3 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

4 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

5 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

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

7 Errors in DACs –Quantization noise, level errors Out of the DAC vs. into the ADC A CD has the samples rounded off to the closest of 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 → Time (jitter) → Level (quantization) Digital audio has two fundamental errors:

8 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

9 Digital audio with USB USB is packet based (1 or 8 packets/ms) –CD audio with 44.1ksps is sent as 9 packets of 44 samples and one packet of 45 –It is hard to synchronize to this scheme!

10 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

11 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/iPad

12 Sample rate conversion 1.The task has 44.1 points per foot of line 2.Draw it up with curve ruler #4 3.On translucent paper: draw 48 points per foot 4.Ask Junior to solve the task

13 Sample rate conversion points / foot = Sample rate of CD 2.Curve ruler #4 = digital FIR filter 3.48 points / foot = Sample rate out 4.Junior = DAC Windows and OS X do this without asking you! This was expensive at the time of DAT. Today it has no cost in and 48 out means: Each 147. input sample == each 160. output sample Every other sample is calculated, and different from the original!

14 Playback tips and tricks Decide ambition level 1.Everyday use Plug’n’play MP3 YouTube 2.Informed use CD quality Tidal / Wimp-HiFi 3.Hi-res Needs a bit more setup >48ksps No “pling, you got mail” Dedicated music player SW 4.Über geek

15 Informed use on Windows

16 Advanced use on Windows –If CD quality is sufficient, use WASAPI protocol –Download ASIO driver from –Use DAC in UAC2 mode (USB Audio Class 2) –Use an ASIO compliant player program foobar2000 with its ASIO plugin JRiver Media Center –Use a virtual audio device JRiver Media Center Hi-Fi CABLE & ASIO Bridge –Read the Henry Audio Blog

17 Informed use on a Mac

18 Informed use on iPad / iPhone –Use latest iOS –Purchase Apple’s "Lightning to USB Camera Adapter“ or "Apple iPad Camera Connection Kit". –Purchase a USB hub with its own power supply –Use DAC in UAC2 mode –Read the Henry Audio Blog

19 Driver summary USB Audio Class 1 = OK for CD quality ● USB Audio Class 2 = Hi-Res, 192 ksps ● Windows OS X LinuxiOS UAC1 Built-in driver with resampling. Works with all software. Built-in driver with resampling. Edit mpd-config. Doesn’t work with UAC1 in current firmware. UAC2 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!

20 Demo!