W A R N I N G !

W A R N I N G !

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Sunday, May 27, 2012

[Beyond the hype] NeoMini PCM2702 USB DAC

A few years ago, USB DACs based on PCM270x series of USB-receiver-cum-DAC were the rage. I can't give you the exact dates because I was late to the party and have stopped going for a while now, but 2007 to 2009 seems like a fair estimate.

USB DACs were being recommended because people believed that -

1) They are external, which means that they are affected less by the PC's noisy power supply and interference compared to an internal sound card.

2) For the same amount of money, a USB DAC would have better stereo playback performance than a sound card, which needs to spend money on surround sound and recording.

Sadly, 1) and 2) are false (I will talk about this later), and I have discovered that 3) Rich gullible people do not know how to open up their computer to install a sound card, and 4) Designing a sound card requires a lot of skill, which the weasels looking for quick profits do not have, so they enter the market with amps and USB DACs and claim that they rock.

Anyway, so people bought USB DACs. And the audiophile products at that time were largely based on PCM270x series of ICs. Before this there were already USB DACs based on Philips and C-Media receivers, but they were low-cost low-price products aimed at adding additional functionality to the computer, not to improve (or degrade) stereo audio performance. Oh and there was M-Audio Transit based on TAS1020, but "it is a pro-audio brand therefore it sucks", and people back then didn't know much about USB and digital audio technology. (They still don't know much now, but whatever.)

It is easy to understand why manufacturers chose PCM270x. It is a USB receiver with a built-in DAC, and PCM2704/5/6/7 even had built-in headphone driver, so all you need to add to make a functional product are some capacitors, some resistors, and a regulator. And also a 12MHz crystal but this is needed for all adaptive-clocking (i.e. cheap) USB devices.

The simplicity also meant that many of these DACs have similar designs, and hence likely to perform similarly. And hence I believe the performance of NeoMini would be representative of similar USB DACs - PCM2702, no op-amps.

Measurements - RMAA

As can be seen above, NeoMini's USB performance fluctuates a lot. This is due to a poor local power supply circuitry that is unable to block noise on the computer's power supply.

So lets talk about myth 1) here. Thing is, the majority of the noise does not travel through the air. But rather, it is conducted, traveling on the power supply wires. So whether the audio circuit is inside or outside the casing makes little difference, as long as it takes its power from the dirty computer power supply.

Secondly, a sound card with a good local power supply circuitry will be able to block whatever noise that is on the computer power supply, even if it is internal. The measured SNR of the good sound cards (-110dB and better) is proof.

Performance vs CPU voltage and frequency

I have previously discovered that the SNR of the onboard sound depends on the load/voltage/frequency of the CPU, and the NeoMini, being poor at rejecting power supply noise, should exhibit the same behavior. So I did the same tests on the NeoMini, and:

It is clear that the NeoMini gets less noisy if the CPU isn't drawing too much power and idling. (The CPU is at 2.8GHz @ 1.325V). But the difference in noise is HUGE!

And if you look at the spectrum:

The green graph is in front of the white, so it would not be blocked by the white graph.

The number and height of the peaks are scary. Many peaks are more than 10dB higher than when @ 800MHz. However due to the high peak at 1kHz for both test conditions, the difference in noise level will not appear to be huge when converted into number form. Okay not really, 6dB is still a huge difference. That's four times the goddamn sound power.

Performance vs CPU load

NeoMini's performance also dips when the CPU is under load. IMD+N is obviously worse, though all other parameters fall within the range of fluctuation.

Detailed RMAA results

I'm going to judge the NeoMini based on a measurement made under the best conditions - 800MHz @ 0.75V idle, to give NeoMini a higher chance to not suck. It needs the help.

NeoMini 16/48

Results discussion

Alright, these are not very good numbers. Under optimal conditions it is about on par with Audigy, And Audigy is a card that should be retired. At least Audigy was good during its days. On the other hand, Audigy was expensive during its days, NeoMini was not.

Interestingly, NeoMini sounds subjectively better than Audigy. Audigy's sound is messy and thin, while NeoMini's is more well-defined. The bass of NeoMini is not strong nor deep and quite bloated, but still sounds nicer than Audigy's, though I'd say Audigy is closer to "normal" in this aspect. (Disclaimer as always: Observations were not double-blind-tested.)

Verdict: Mixed

With measured performance the same level as onboard sound of that time, the only reason you'd want to buy this is "because it sounds better". Fortunately, it does, at least I think so.

I'm going to talk about myth 2) as promised. You see, price and cost are not directly-related. Manufacturers sell things according to what they think the value is, and both manufacturers and consumers feel that surround sound and recording functions are worth little. So they will still charge a lot for stereo-only, because everyone think it is worth it.

Also, while one may argue that some money of sound card goes to the surround and recording, I am reminding you that a large part of the cost of DACs/amps goes to the enclosure. NeoMini doesn't have an enclosure so it was much cheaper than completed alternatives for the same performance.

And in reality, the most amount of money goes to the design. Or how much the designer thinks his design is worth because buyers have no way of measuring the worth of a design.

So for example, NeoMini's price in Taiwan is IIRC ~$35 Singapore dollars before shipping. Onboard sound is at most a few bucks. And both perform similarly. But you would want to buy the NeoMini USB DAC, because you think it sounds better than onboard sound.

1 comment:

Gaston said...

Hey thanks, you got me a little wiser!!