After doing a first research on this topic on the internet I realized that this won’t be very easy as most of the manufacturers just don’t put any useful info regarding frequency response on their webpage. And if you asked them they, often just don’t know it themselves.

So I ordered these interfaces for testing:

• M-Audio FastTrack Pro (about €160)
• TASCAM US-122 MKII (about €130)
• E-MU 0202 USB (about €100)
• ESI Dr. DAC nano (about €70)

Here are the results of the individual audio interfaces.

M-Audio FastTrack Pro

The problem with this interface was that it still uses USB 1.1 and therefore a sampling rate of 96 kHz is only possible in one direction (in or out). That would be OK for me as I only needed to use it for output, but whatever I tried I did not get the interface to work with a sampling rate of 96 kHz – it only accepted 44.1 kHz. As a result the upper end of the frequency response is around only 20 kHz. So I cannot use this interface for my purpose.

TASCAM US-122 MKII

This was the best interface in respect of frequency response in my test: in contrast to the FastTrack Pro it uses USB 2.0 and therefore can easily handle 96 kHz sampling rate in both directions simultaneously. The frequency response is also very good (20 Hz to 40 kHz with +/-1 dB) as the following diagram of the RightMark Audio Analyzer shows:

E-MU 0202 USB

Whatever I tried, but I did not get this interface to work correctly: my Windows XP computer crashed several times while trying to get the drivers to work and I really don’t want to talk about the Windows 7 Beta Drivers at all! I cannot understand why a company like Creative (E-MU was acquired by Creative in 1993) does not have official (not beta!) and working Windows 7 drivers about 2 years after this operating system was released.

ESI Dr. DAC nano

This was the cheapest interface in my small test and it also does not have many features: it really only outputs stereo audio – over S/PDIF or line out –, but this works just fine at 96 kHz sampling rate. The frequency response is not what I would call “neutral” (it looks like as the manufacturer want to boost the bass a little bit with intent), but at least also the higher frequencies are sent with not too high absorbability (20 Hz to 40 kHz with +2/-3 dB). For this test I used the TASCAM US-122 MKII interface as input for the RightMark Audio Analyzer:

As a summary I recommend the ESI Dr. DAC nano interface if only stereo output at 96 kHz is necessary and +2/-3 dB in the 20 Hz to 40 kHz frequency range are OK. Otherwise the best one in respect of frequency response of the tested devices definitely is the TASCAM US-122 MKII.

Let’s start with a code listing and then I’ll explain what I’m doing here:

const double frequency	= 1000;
const double amplitude	= 20000;
const long sampleRate	= 44100;
const int durationSec	= 5;

long sampleCount = sampleRate * durationSec;

double timeStep = 1.0 / (double)sampleRate;

double time = 0;
int[] values = new int[sampleCount];
for (long i = 0; i < sampleCount; i++) {
	values[i] = (int)(amplitude * Math.Sin(2 * Math.PI * frequency * time));
	time = time + timeStep;
}

OK, here are some explanations:

• lines 1-4: some constants you can change to e. g. adjust the frequency.
  Note: the frequency cannot be more than half the sampling rate.
• line 6: calculating the number of sampling points.
• line 8: calculating the time between two sampling points.
• lines 10-11: some variable initializations.
• lines 12-15: here finally the value of each sampling point is calculated.

And here is the corresponding code for Visual Basic .NET:

const frequency as double		= 1000
const amplitude as double		= 20000
const sampleRate As Long		= 44100
const durationSec As Integer	= 5

Dim sampleCount As Long
sampleCount = sampleRate * durationSec

Dim timeStep As Double
timeStep = 1.0 / sampleRate

Dim time As Double = 0
Dim values(0 To sampleCount - 1) As Integer
For i As Long = 0 To sampleCount - 1
	values(i) = amplitude * Math.Sin(2 * Math.PI * frequency * time)
	time = time + timeStep
Next i

For sound playback you can now use either an API call like PlaySound (of winmm.dll) or the solution of this great article.

Did you also need to generate a sine in .NET yourself already?