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Analog Research-Technology
Electronics for Serious Audiophiles



The scoop on the Legato USB-SPDIF Asynchronous Converter


OK, seems that everyone is making a USB-SPDIF converter. True, but not many of them are capable of asynchronous operation.

"So, what is so great about async operation?"

To put it in the simplest terms, in asynchronous operation, the converter box contols all of the timing. The typical converter is not capable of controlling the timing. To make the situation worse, they let the computer supply the clock. It is essential that a clock of the lowest possible jitter be used to control the timing. And we all know that you aren't going to find anything quiet enough inside of ANY computer.

The Legato clock:

Not only does the Legato provide the clock, it also has one of the lowest jitter clocks in the industry. (We would like to say that it is the lowest jitter, but there is always a chance that someone makes one as quiet as ours. Not likely, but possible. No one can ever accuse of of being disingenuous.) Sure, there are units that have an internal clock, but do so with an off-the-shelf oscillator module. In fariness, some are fairly decent, others are not. But "fairly decent" is not good enough for us.

In order to have a low-jitter clock, you need to have a quiet power supply. Most units let the computer supply the power, as well as the clock. Most people would agree that computers don't have very quiet supplies.

The Legato power supply section:

The Legato has its own internal power supply, along with a dual-tier regulation scheme. Each regulator stage is a low noise design, made with carefully selected discrete components. A lot of designers would be content to use whatever 3-terminal regulator fits nicely on the PCB. But that is not good enough for us. Yes, our appraoch does cost a bit more, both in terms of cost and parts count. But it is essential that the regulators be very quiet in order for the clock to perform with the lowest possible jitter. In fact, the bulk of the PCB houses power supplies. Compare that to units that either allow the computer to supply the power, or use a "wall wart". A serious product should have a serious power supply. The Legato has serious power supply. A very serious power supply.

"OK, sounds good so far. But what else is special about it?

Well, the other part that is very important to performance:
The Legato output stage.

The Legato output stage uses a high-quality pulse transformer. This provides for complete galvanic isolation from the DAC. No ground loops to lower performance. But that is only part of what makes the Legato better. The output is reclocked, prior to the transformer. This eliminates the inherent jitter that is present, with any complex digital "chip". The output signal then goes to a true 75 ohm BNC connector. Typical units use RCA connectors. RCA connectors are not a 75 ohm connector, and this creates performance-robbing reflections. (OK, some some people will claim that their RCAs are "75 ohm RCA connectors". Just because they claim it does not make it true. Because it is not true. Despite what their marketing department says.) You will not find an SPDIF output that adheres as closely to 75 ohm impedance, and to over a wider frequency range. A much wider frequency range than even the SPDIF signal itself. Our extensive engineering backgorund in RF allows us to not only design perhaps the best clock in the industry, but more likely the most accurate output stage available.

"Yeah, all sounds impressive, but don't you guys know that almost the entire world uses RCA connectors. Even if they are inferior, that is what my DAC uses. So, what do I do?"

Well, the Legato comes with 2 accessories to addresss that issue. We provide, as part of the product, a SPDIF cable with BNC connectors on both ends. For those of you with a high-quality DAC that uses BNC connectors, you simply hook up the supplied cable, and you are set. But this is not a cheap "throw away" cable. No, sir. This the same cable that came as standard with our old Segue D/A converter.

When we built the Segue D/A converter, we inherently realized that our customers needed a high-quality cable, in order to get all the performance we designed into it. We utilized our extensive knowledge of RF, in order to come up with the right cable. We included this cable, that yielded the level of performance necessary, as a service to all of our customers. This not only saves the customer money, but also the hastle of trying a seemingly endless number of cables, trying to get the maximum performance. This same philosophy has carried over to the Legato For that reason, we include it with the purchase of every Legato.

And for those of you who have a DAC that has RCA inputs, we supply a BNC-RCA adaptor. Simply put the adaptor on the provided cable, and plug it into your DAC. Yes, there will be some performance degradation when using an adaptor, but the nature of the provided cable will help to overcome that problem.

"Hmmm, this is sounding better to me now. What else can you tell me about it? What about sampling rate?"

The Legato only works with 44.1 kHz sampling rate. Yes, we know that a lot of you want hi-res capability. We could provide that, but it would sacrifice performance. Multiple sampling rates would require mulitple clocks, all of which would need to be turned on and off as needed. Each with its own very carefully designed clock distribution scheme. While we could provide these features, we decided to build the very best 44 kHz unit that we could, without compromise, and yet at a very affordable price.

We designed the Legato with folks like us in mind. Folks who have hundreds, perhaps thousands, of CDs in their collection. Folks who demand the very best performance from their existing music library. Folks who are not willing to accept compomises in performance, made for the sake of flexibility. The Legato was designed for the traditional CD market.

And let's be honest: most of us have very little hi-res source material. Even when more hi-res material becomes available, are you really going to buy Dark Side of the Moon, if it comes out in hi-res?

Again!?

No, probably not. Many of us have ripped all of our CDs onto a HD, and want the best possible sound from what we already have. That is what we demand. And that is the segment of the market that we hope to please with the Legato.

That is about it. Other than the price. Which is only $500.

So, you can buy a nifty-looking doo-hickey that will almost fit into your shirt pocket.

That draws its power from the computer.

And uses the computer's clock.

And not much more. Especially in the way of all-out performance. Or................

You can take the same amount of money and plunk it down on a no-compromise unit.

With its own power supply.

And a damn good clock.

And a really damn good output stage, that is transformer coupled.

And comes with a high-performance cable.

All for only $500.

We know which one we would chose! But then, we are biased.

"OK, I'm almost sold, but what kind of computer will it work with? Do I need to install drivers? Are some types better than others?

Good question. The Legato will work with any modern Windows, Mac, or even Linux computer. Your computer will have the necessary drivers. The only concern that one might have will be choice of operating system. Our testing, coupled with the feedback of our early users, is that Macs yield the best performance. However, some folks who have been lucky enough to get "7" to work, have reported results comparable to the performance of Macs.


For information, or to order one:





Legato specifications   





For those of you who want some more detailed technical information, here is some
"Geek Speak"
   





 
 

 

 

ANALOG RESEARCH TECHNOLOGY: Years of design experience in all phases of analog electronics, ranging from DC to microwaves.

 

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