Latency.md

Latency

The default latency of the driver is set to 4.2ms. This is the 'high quality' setting, to ensure that not even the highest jitter in the USB system affects the audio quality. However for some applications, low latency is more important than an occasional click in the audio. To adjust the latency to 1.0ms (good low latency value) you just select "y" when the installer asks Use low latency setting?.

Latency Details

The total latency is the sum of (1) the driver latency, (2) your application's latency (buffer setting) and (3) the EMU internal latency. The EMU one-way internal latency typically is 1.3 ms (96k) to 1.8 ms (44k), the exact value depends on the selected sample rate and the type of EMU device that you have. The roundtrip will be twice this. If you select the low latency driver and e.g. a 64 sample buffer in your application, this gives you around 5.9 ms (96k) to 8.5 ms (44k) total roundtrip latency. The driver reports the estimated latency to the system, and eg reaper shows it in its menu bar (including its own buffer latency).

Here is a table with measured roundtrip latencies in milliseconds. The driver was installed with the low latency setting. The values N=128 and N=64 show the DAW buffer size.

rate	latency N=128	latency N=64
44k	11.6	8.5
48k	10.8	8.1
96k	7.3	6.0
176k	5.6	4.9
192k	5.5	4.9

How is it measured?

To measure roundtrip latency you need reaper (free trial download from http://www.reaper.fm/ ) and a loopback cable for the EMU. The cable loops the EMU output (either TS jack or 1/8" stereo TRS jack) into a line input ( 1/4" TS jack)

1. Attach the loopback cable.

2. The direct monitor must be off. Phantom power must be off.

3. The input and output volumes adjusted so that the signal comes back in approximately with the same volume.

4. Unzip this click track, unzip and load it as reaper track.

5. Make a 2nd track to record it on (Track menu/Insert new Track) and press the record icon to arm the track for recording.

6. Set the input channel for the 2nd track to the channel that your loopback cable comes in on (or record both tracks)

7. go to prefs. the easiest way is from the menubar where it shows the sample rate, select audio device settings in that menu, and set the settings you want, then in the left hand side of that dialog box go down to "recording" and uncheck "use audio driver reported latency" and make sure the offset fields are also zero.

8. press record - the audio from track one should go out the output and through the loopback cable and into track two.

9. zoom in and see how far the recorded signal is behind the original. this is the total latency including the daw buffer.

It can help to see the latency by selecting a time period by dragging with the mouse over a empty bit of screen below the tracks

You can set the time display units by right clicking on the time display at the bottom right of the screen.

Further optimization

If you really need the lowest possible latencies, you might consider hacking the installer before running it and replace the 1000 (microseconds) in there with an even lower value. You may have to do you own measurements (see #40) to see how far you can go without distortions. For the 0404 these seem absolute minimum values:

rate	min latency
44k	0.57ms
48k	0.58ms
88k	0.70ms
96k	0.70ms
176k	0.58ms
192k	0.740ms

Comparison with other cards

The following table shows the EMU performance. All values with 128sample DAW buffer. All latencies are the total roundtrip latency in milliseconds.

device	latency @44k	latency @48k	latency @96k	latency @192k
Windows7 EMU0404 PCIe	6.1	-	-	-
RME Fireface 800	9.7	8.9	6.0	5.5
Windows M-Audio 1010lt	10.3	9.5	4.7	-
Windows7 Presonus Firestudio Mobile	10.3	9.6	6.1	-
Alesis Photon 25	10.34	10.33	-	-
EMU0404 (low latency setting)	11.5	10.8	7.2	5.4
Scarlett 2i2	11.1	12.5	-	-
Native instr Kore 1	12.4	11.9	7.1	-
Behringer FCA202	13.5	12.0	7.7	-
PreSonus Studio 192	14	-	-	-
Windows7 Line 6 GX	19.3	18.1	-	-
Windows7 Creative Sound Blaster X-Fi pci-e	20.4	19.2	19.4	-