MEASUREMENTS: Tascam UH-7000 USB Interface (Part II: As an ADC)

TEAC & Tascam combo with size differential. The E-MU 0404USB in the background to the left.

Alright, it's time to spend some time looking at using the Tascam UH-7000 as an analogue-to-digital converter. Remember in the first instalment, I showed that this device is already a very good DAC. This time around, I wanted to see whether this device did the job well as an ADC; my main desire being to use it for vinyl needle drops and to see if it can be used as a good measuring device in comparison to the Creative E-MU 0404USB I've been using for the last couple years (which I bought mainly as a DAC back in 2009).



In terms of the underlying hardware, the Tascam UH-7000 uses the Burr-Brown PCM4220 ADC compared to the E-MU's AKM AK5385A chip. On paper at least, the Burr-Brown should be significantly better with a rated SNR of 123dB compared to the AKM's 114dB. Of course, much of the final result depends on the circuitry built around the ADC such as quality of the pre-amplifiers feeding the input signal.

I. The Spectrum of Silence

To start, let's have a look at what "silence" looks like through the Tascam UH-7000, with preamps set to minimum:

Impressive. With the ADC running at 24/96, we're seeing very low noise floor essentially flat down to -160dB across the spectrum (Note: I found an issue with this later on - see below).

Compared to the E-MU (sorry about the difference in scale!):

Clearly the old Creative unit is noisier with sporadic noise spikes reaching up to -140dB and a slight tendency for the noise floor to rise from about 35kHz and up.

So far, so good... The Tascam is doing well!

Let's now run a few RightMark tests to see how some of my DACs measure using both the Tascam and E-MU. I'll be looking at the AudioEngine D3 USB DAC as an example of a very capable but "lower tier" DAC in terms of resolution and the TEAC UD-501 as an example of a higher-end desktop DAC which would likely challenge the resolution of these ADCs. This will provide an opportunity to correlate the results between different "instruments". If the Tascam results closely follow what I've been seeing with the E-MU over the years, then at least it's suggesting that I'm on the right track with these measurements :-).

Basic setup:
Windows 8.1 Surface Pro 3 --> 6' Shielded Belkin Gold USB --> DAC [AudioEngine D3 / TEAC UD-501] --> analogue cable (shielded RCA for D3 / XLR for TEAC) --> ADC device [Tascam / E-MU] --> shielded USB --> measurement Windows 7 laptop

Tascam latest Windows driver: 1.01
Tascam latest firmware: 1.07

ASIO (TEAC) or WASAPI (AudioEngine) drivers used for playback.
ASIO for all recording.

RightMark 6.3.0 used as measurement suite.

II. RightMark Comparisons

     16/44:

As usual, let us start with the most common audio resolution - good old CD-quality 16/44. Here is an overall score sheet:

A few graphs to consider:

16/44 Frequency response: Essentially flat.

16/44 Noise floor: About the same across the board.

16/44 THD: Notice a bit more "skirting" at the base of the primary signal for the D3 suggesting more jitter as compared to the TEAC. Both Tascam and E-MU consistent in picking this up.

16/44 Stereo crosstalk: Interestingly, the AudioEngine has lower crosstalk than the TEAC despite the TEAC using XLR cables. Both Tascam and E-MU consistent also in this finding.

As you can see, 16/44 is really no challenge at all for these DACs (D3, TEAC) and measure essentially identically using both ADCs (Tascam, E-MU). The results show essentially ideal measurements. Not surprising that in the 2010's, reproducing a CD-resolution signal is really a "piece of cake" for reasonable quality digital audio gear.

     24/96:

Time to delve into the world of high-resolution with 24/96 then. Here's the summary:

Okay, a little more variance this time around compared to 16/44. However we basically see that the TEAC is capable of better noise level and in turn dynamic range compared to the AudioEngine D3 DAC (about an extra 1/2 bit or 3dB better for the TEAC). Interestingly, the E-MU actually turns in slightly better numbers in both noise floor / dynamic range as well as composite distortion numbers than the Tascam. Let's have a look at the graphs then:

24/96 Frequency response: In terms of the DAC, the TEAC has a flatter more extended frequency response as demonstrated with the Tascam measurement. It's interesting that the E-MU tends to roll-off the ultrasonic spectrum >20kHz very slightly (<1dB difference at 40kHz).

24/96 Noise floor: Not much difference really... TEAC slightly lower and this is consistent for both the Tascam and E-MU measurements.

24/96 THD: More evident than with the 16/44 measurement above, there's more "skirting" with the AudioEngine suggesting perhaps more jitter. But what's that peak at 30-40kHz showing up on the Tascam measurement?

24/96 Stereo crosstalk: Again, interestingly less crosstalk measured with the AudioEngine D3 and this is again consistent with both Tascam and E-MU as measurement devices. Hmmm... There's that odd spike at 30kHz again with the Tascam measurement of the TEAC DAC.

Despite a bit more variance, we're basically seeing consistency in the measurements using the Tascam and E-MU devices. However, I noted that unusual spike in the Tascam measurement of the TEAC UD-501 at 30-40kHz. How odd... We'll look at this in greater detail in a bit.

III. Dunn Jitter Test Comparisons:

So far, RightMark results are reasonably consistent and there's agreement between the results from my E-MU and the new Tascam unit in terms of how the two DACs measure. How then do the J-Test results look?

AudioEngine D3:

TEAC UD-501:

Unfortunately I pushed the volume a little higher with the Tascam on these UD-501 screen captures. As a result you can see noise spikes like the 10kHz spike with the E-MU 24-bit J-Test show up in a more obvious fashion (refer to the image above of the frequency spectrum of silence with the E-MU).

In any case, I think it's quite evident that there's not much discrepancy between the Tascam and E-MU. Both the AudioEngine D3 and TEAC are demonstrating rather good J-Test spectra, consistent whether using the Tascam or E-MU as ADC "measurement" devices.

IV. Tascam, we have a problem...

Remember up above when I looked at RightMark results with the Tascam and saw that usual ultrasonic noise above 30kHz with the 24/96 measurements of the TEAC?

Here's what I found (these are all from the left channel). When I start the Tascam "cold", there doesn't seem to be a problem:

Tascam UH-7000 24/96 "silence" at startup. Beautiful!

Tascam UH-7000 24/96 "silence" 15-minutes turned on. Couple little oddities.

Tascam UH-7000 24/96 "silence" 30-minutes turned on - LEFT channel. Huh? What's that?

As you can see, as the machine "warms up", noise starts creeping into the ultrasonic range between 30-40kHz! Swapping USB various cables made no difference nor did changing computers (Acer laptop and Surface Pro 3 tried). Of course I also made sure the Tascam wasn't situated close to another device which could be causing this noise. I even tried it with my old MacBook Pro and saw the same problem with OS X drivers.

I also played with the various settings of the device mixer itself to make sure I didn't accidentally turn on some DSP effects but to no avail.

BTW, here's a look at what the mixer panel looks like with knobs and buttons for effects like the compressor, noise suppressor, EQ, limiter...:

Finally, I plugged it into my Belkin power conditioner which made no difference so I am left with the conclusion that the noise is originating from within the device itself.

The noise isn't as "loud" with the right channel but obviously still there:

Tascam UH-7000 24/96 "silence" 30-minutes turned on - RIGHT channel.

As best I can tell, this seems to be something that happens as it warms up. In cooler ambient temperature (like my basement), it takes longer but will eventually show this noise pattern. If you look at the 24/192 FFT, it looks like there's quite a bit of noise up at 60-80kHz as well (maybe the 30-40kHz noise is a subharmonic):

Tascam UH-7000 24/192 "silence" 30-minutes turned on - LEFT channel.

I wonder whether this is noise from the switched-mode power supply (SMPS) given the close proximity to the audio circuits and whether this could be suppressed with better line filtering or RF shielding of the power supply. Well, here is demonstration of one piece of audio equipment where the audiophile practice of "warming up" the gear actually deteriorates performance.

V. Conclusions:

Since I bought this unit from Amazon, I had the opportunity to return it hassle-free... And that's what I ended up doing after putting it through its paces over about 3 weeks. I made a 24/96 vinyl needle drop of the Paul Simon Graceland album (Side 1 - same as what I did for the LP Test a couple months back). No question, it sounded good; punchy dynamics, excellent details, sounds just like the original LP playback. The ability to fine-tune the level controls with those large knobs on the front panel and the beautiful LED indicators to monitor for clipping will certainly be missed!

Throughout the test period, I never ran into any crashes with the driver software (unlike the issues I have with the E-MU on occasion). I also like the aesthetics of the device - goes well sitting on top of my TEAC UD-501.

I don't know if this particular unit was defective but I've received feedback from a friend in Europe with the UH-7000 that he's seeing noise in his unit as well:

European Tascam UH-7000.

If this is a systemic issue, I certainly hope Tascam fixes this perhaps in future hardware revisions (hopefully it's just a firmware/driver issue)... I'm very happy with Amazon's return policy and I must say I'm quite impressed by the little E-MU 0404USB workhorse despite its age and buggy drivers that will on occasion hang with samplerate changes. The Tascam has demonstrated to me that the E-MU ADC tends to roll-off the ultrasonic frequencies slightly (only -0.75dB up around 40kHz) and the noise floor is obviously not as clean as it could be (perhaps related to the little wallwart power supply).

Ultimately, the noise I found with the Tascam ADC once it warms up is of low-level and would not be audible in regular use (certainly not audible with LP needle drops, but high-resolution multitracking studios and those using effects processors might care) due to it being ultrasonic and down at below -100dB (a subjective reviewer would not have been able to pick this up). However, it is an unexpected finding which does show up when I do high samplerate measurements. If it were not for this, the unit would have stayed as a fixture on my audio equipment rack in the sound room.

Can anyone recommend to me a good ADC that is highly accurate, reasonably priced (maybe $500-600USD), stable Windows drivers, has good level controls, and provides good visual feedback of volume levels to avoid clipping? I'm certainly happy to purchase a Tascam unit like this one again in a few months and see if this issue is gone if by then I haven't found a better ADC.

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I hope everyone had a wonderful Christmas and New Year time! Time to get back to a hectic work and family life... May your 2015 overflow with good music :-).