low-latency audio I/O for Windows: a report

["Freed, Dan" <DFreed@xxxxxxx>]

Dear Auditory List Members:

In October I posted a request for information about low-latency audio
interface devices for use with Windows.  I received many helpful
responses.  Over the last two months I've had the opportunity to acquire
several devices and measure their latencies.  Since latency information
is generally not reported (or is incorrectly reported) in manufacturer
specifications, I'm posting my measurement results here.

By "latency", I mean the total end-to-end delay imposed by the device
and its driver, from analog input to analog output.  This doesn't
include any additional delay imposed by the software signal processing
(filter group delay, FFT blocking delay, etc.).

Latency was measured by presenting a pulse train to the analog input,
viewing the analog input and output on a dual-trace oscilloscope,
comparing the traces, and visually estimating the delay.  Tests were
performed under Windows XP.  The PC was running a simple 1-channel
input-to-output copying program that accesses the device through the
ASIO driver interface.  Each device was tested at multiple sampling
rates.  At each sampling rate, testing was performed using the shortest
buffer length that the device supports for that sampling rate, so the
measurements are best-case results.

Results are shown below.  Sampling rates are in kHz, latencies are in
ms.  Buffer length in samples is shown in parentheses.  Note that
latencies under 3 ms are achievable at the higher sampling rates with
some devices.

EDIROL UA-1EX [USB device, $80]
32 kHz: 14.2 ms (96)
44.1 kHz: 11.5 ms (96)
48 kHz: 12.0 ms (112)

M-AUDIO FIREWIRE SOLO [FireWire device, $172]
44.1 kHz: 9.0 ms (64)
48 kHz: 8.2 ms (64)
88.2 kHz: 6.5 ms (64)
96 kHz: 6.1 ms (64)

ECHO AUDIOFIRE 4 [FireWire device, $300]
32 kHz: 8.0 ms (32)
44.1 kHz: 6.0 ms (32)
48 kHz: 5.5 ms (32)
88.2 kHz: 3.6 ms (32)
96 kHz: 3.4 ms (32)

RME FIREFACE 400 [FireWire device, $1000]
[at some sampling rates, 48-sample buffer caused bus errors, so used
64-sample instead]
32 kHz: 10.4 ms (64)
44.1 kHz: 6.6 ms (48)
48 kHz: 6.0 ms (48)
64 kHz: 6.2 ms (64)
88.2 kHz: 4.5 ms (64)
96 kHz: 4.2 ms (64)
128 kHz: 4.2 ms (64)
176.4 kHz: 2.9 ms (64)
192 kHz: 2.5 ms (48)

M-AUDIO DELTA 44 [PCI device, $200]
16 kHz: 16.2 ms (64)
22.05 kHz: 11.7 ms (64)
24 kHz: 10.7 ms (64)
32 kHz: 8.1 ms (64)
44.1 kHz: 5.9 ms (64)
48 kHz: 5.5 ms (64)
88.2 kHz: 3.0 ms (64)
96 kHz: 2.7 ms (64)

ECHO LAYLA 3G [PCI device, $500]
32 kHz: 8.8 ms (64)
44.1 kHz: 6.4 ms (64)
48 kHz: 5.9 ms (64)
64 kHz: 4.2 ms (64)
88.2 kHz: 3.1 ms (64)
96 kHz: 2.8 ms (64)

RME MULTIFACE II + HDSP PCI [PCI device, $1049]
32 kHz: 6.5 ms (32)
44.1 kHz: 4.7 ms (32)
48 kHz: 4.3 ms (32)
64 kHz: 3.8 ms (32)
88.2 kHz: 2.8 ms (32)
96 kHz: 2.5 ms (32)

Caveat: running at high sampling rates with short buffer lengths
increases the risk of dropouts.  I did some limited listening tests in
all of the above testing conditions and never heard any dropouts, but I
offer no guarantees.

I'd be happy to answer any questions about my measurements.  I hope this
information is useful.

Dan Freed
Senior Engineer
Dept. of Human Communication Sciences & Devices
House Ear Institute
2100 W. Third St.
Los Angeles, CA  90057  USA
Phone: +1-213-353-7084
Fax: +1-213-413-0950
Email: dfreed@xxxxxxx