[audioengr]

Quote:

Originally Posted by Rivendell61 /img/forum/go_quote.gif Dan Lavry tried to explain that to you last year on his forum. Here are some Lavry quotes: "....RCA or just about anything out there....is somewhere between say 25 Ohms and 180 Ohms....that one inch discontinuity at the middle of a line with a signal of say 5 nsec rise time is virtually a non issue." And more Dan lavry: "I agree that an RCA connector, (if it is far from 75 Ohms impedance), would be a disaster for 10GHz digital signal transmission. It is certainly NOT a problem for say audio signals because they are too slow. .....AES and SPDIF signals are still way too slow to care about the impedance of an RCA connector." "An SPDIF signal DOES NOT have spectra to GHZ." Also--Here on HeadFi Mr Siau has addressed the question in this post: http://www.head-fi.org/forums/showpo...68&postcount=3 Mark

Mark,
I will have to differ with Mr. Lavry, who I have met and discussed many things with. I consider him a peer, as I do Mr. Siau. There is significant spectra approaching GHz in a proper S/PDIF signal (one that generates low-jitter at the receiver). Just display the spectra on a RF spectrum analyzer and you will see it.

Many DAC's have significant filtering to slow the S/PDIF edges which does reduce this spectra. This is a bad idea IMO. Faster edges result in less jitter at the receiving device, so this is more desirable. You do have to take more care in terminating the transmission-line properly and impedance matching, but this is not rocket-science, at least for me. I have 30 years experience with high-speed digital interfaces, from IBM peripheral buses to massively parallel supercomputers. Rise-times of 2 nsec on cables are not unusual in these systems.

What you must understand is that no one designer has infinite knowledge of all aspects of these designs. Some are strong in digital, some in analog. Few have much experience in power delivery, ESD, grounding and shielding and even less have much experience in HF digital transmission line effects and termination techniques. I am strong in certain areas and these designers may be strong in other areas. I would not try to compete with John Curl for instance to design a power amplifier circuit. However, I can improve the current paths and power delivery to his amp and make it sound even better. This is why I have found a niche in the modding business.

I find all kinds of crazy things in some DAC's that I have modded. Some designers have limited experience and therefore just copy other designs which also may be non-ideal. I have found poor implementations of USB interfaces and S/PDIF interfaces in lots of products, even from large companies like Sony. I'm not your typical modder that swaps parts. I have extensive experience in design, so I reverse-engineer most designs to expose the weaknesses.

The point is: there are no experts in everything. We are each experts in certain areas. This is why collaborations often lead to superior products.

Steve N.

 

[Crowbar]

Jocko and others on the diyhifi forums actually performed measurements that show there is definitely a concern with proper connector impedance. Instead of arguing, let's see some properly conducted time-domain reflectometer experiments and settle this.

Of course, the true solution is drop synchronous modes altogether. There's a reason the USB Audio standard supports asynchronous endpoints where the DAC provides flow control, and thus interface jitter becomes a non-issue without any ASRCing and its rate-estimation problem. The latter is explained by Bruno Putzeys in another forum:
Quote:

Any instabilities in the ratio signal will get encoded as phase modulation into the output. Such instabilities may stem from jitter. The whole jitter attenuation capability of an ASRC hinges on the low-pass filter. The jitter attenuation characteristic equals the frequency response of the ratio estimator's post filter. This filter fulfils the same function as the loop filter in an analogue clock recovery PLL. The advantage of the ASRC is then that you need only one crystal oscillator to cover all sampling rates. Of course the ratio estimator, being an implicit ADC, suffers from quantisation. The phase between the two clocks is quantised to a time span equal to 1 period of the highest frequency clock in the chip (sometimes a multiple of the output rate, sometimes a separate master clock signal). This error is added to the input jitter before being attenuated by the lowpass filter.

Note that it is clear from the above that rate estimation is not a purely digital process.

 

[Jetlag]

A quick round of kudos to both Elias Gwinn and Rory Rall at Benchmark. After being informed that my DAC1 USB had arrived sans power cord they sent me one in just 4 business days! Excellent CS and much appreciated, thanks fellas!

(BTW, it wasn't necessary guys but thanks for the Nordost Valhalla, I really was only expecting a stock cord.

)

 

[EliasGwinn]

For those following this discussion, I will quote John Siau's previous response to Steve's question (thanks for the link Rivendel61!!)

Quote:

Originally Posted by jsiau /img/forum/go_quote.gif The connector is 50-Ohms. 50-Ohm connectors are far more durable than 75-Ohm connectors due to the extra dielectric material surrounding the center pin of the BNC. For this reason, it is common practice to use 50-Ohm BNC connectors in 75-Ohm systems when the signal bandwidth allows it. The short interruption of the 75-Ohm transmission line is only significant for frequencies that are much higher than any contained in a digital audio signal. The 50-Ohm connector would be a factor for signals having a wavelength of 2 inches or less in coax (about 3 GHz). A 192 kHz digital audio signal transmits data using a clock that is 64 times the sampling frequency (192 kHz * 64 = 12.288 MHz). 3 GHz is the 244th harmonic of 12.288 MHz and does not exist in a 192 kHz digital audio signal. If it did, the box probably would not pass FCC and CE emissions tests. Changing the connector would reduce the durability of the product and would have absolutely no effect even at 192 kHz.

 

[EliasGwinn]

Quote:

Originally Posted by Jetlag /img/forum/go_quote.gif A quick round of kudos to both Elias Gwinn and Rory Rall at Benchmark. After being informed that my DAC1 USB had arrived sans power cord they sent me one in just 4 business days! Excellent CS and much appreciated, thanks fellas! (BTW, it wasn't necessary guys but thanks for the Nordost Valhalla, I really was only expecting a stock cord. )

Jetlag,

Thank you for your graciousness and patience with our oversight. I'm glad we could straighten everything out.

Thanks,
Elias

 

[audioengr]

Quote:

Originally Posted by EliasGwinn /img/forum/go_quote.gif For those following this discussion, I will quote John Siau's previous response to Steve's question (thanks for the link Rivendel61!!)

Thanks for the explanation. I have been putting 75 ohm BNC's on DAC's for years and never had any damage yet, however I too dont believe that the edge-rates are fast enough to warrant this precise impedance matching, particularly for stock consumer gear.

My modded gear is a different story. I get very high-speed edge-rates (not fundamental frequency) on my S/PDIF and so the spectra goes much higher. You can have a 100Hz digital signal with sub-1nsec edge-rate that will have significant GHz content. This is the difference between RF and digital transmission. Talking about the fundamental frequency without specifying the risetime is certainly incomplete. They are totally different animals due to the non-sinusoidal waveform of digital.

I suppose this is like any other detail in audio. It is a matter of what is audible. There is certainly a lot of debate on this and I'm personally not convinced that even an RCA connector makes an audible difference in a typical stock piece of gear. There is some magnitude of reflection with all impedance discontinuities. The question is whether it is audible or not......

I believe the answer is:
It depends on a lot of other factors, the edge-rates, the cable impedance, the terminations etc...

Steve N.

 

[music_man]

so once you modify the gear, material that normally only birds can hear suddenly become audible to humans? i want it!

music_man

 

[audioengr]

Quote:

Originally Posted by music_man /img/forum/go_quote.gif so once you modify the gear, material that normally only birds can hear suddenly become audible to humans? i want it! music_man

I'm not sure what this comment means, but yes, the HF extension, dynamics and clarity is improved dramatically. Improving power delivery, signal and power paths and reducing jitter accomplishes this. You can read the feedback from my customers on audiocircle, audiogon or asylum.

Do birds actually hear HF better than humans? I know dogs do...

Steve N.

 

[Crowbar]

Quote:

The inner ear actually has nerve receptors for frequencies up to about 80 kilohertz.

Source: US patent 5479168

Peer reviewed study showing subconscious perception of post-20 kHz sound:
Inaudible High-Frequency Sounds Affect Brain Activity: Hypersonic Effect
The Journal of Neurophysiology Vol. 83 No. 6 June 2000, pp. 3548-3558

Though we now have 96 kHz sampling rates so digitally 40 kHz sounds should be doable, I'd love to see, in light such information such as what I've referenced above, what excuse DAC makers have about analog filters starting attenuation pretty close to 20 kHz. The lack of higher bandwidth recordings is not an excuse, since it's the other way around: no one would make such recordings unless playback devices supported them.

 

[music_man]

as far as i know most adult humans can not even detect 20khz during the standard hearing test audiologists use.

i am neutral on the subject of modding. i can see why the manufacturers would frown upon that though. for a lot of reasons. thats all i have to say on that subject.

music_man

 

[Jetlag]

Is it just me or did anyone else notice that the title of this thread was "Benchmark DAC1 now available with USB" and not a sponsored promotional thread for advertising modding services?

Elias has offered an exceptional free tool for all of us computer audio fans by creating a WIKI dedicated to maximizing the performance and quality of our audio playback. John has also gone above and beyond by answering many of the more technical questions and IMOHO has done a very thorough and accurate job of doing so. Rory also helped me out by providing great customer service.

I am most definitely an AV enthusiast but because of my tight budget there is no way I can spend over $1K on an item and then voluntarily terminate the manufacturers support for that product. I specifically chose the DAC1 USB not only because it is such a carefully designed, well supported and very highly regarded product, but also for the reasons outlined above. Personally I would never consider voiding the warranty but your own opinions may vary.

No offense intended, but I think if you want to promote your modding services there is an established 'members of the trade forum' just for that purpose. Just my $.02 of course and I hope this isn't considered inflammatory. Thus far this thread has been a cornucopia of useful information for DAC1 USB fans/owners with a rarely seen level of manufacturer participation. I'm not a Benchmark employee or rep, just a satisfied owner who would prefer to see this valuable thread stay on topic. My sincere apologies if I offended anyone by my comments.

 

[lowmagnet]

Quote:

Originally Posted by Jetlag /img/forum/go_quote.gif No offense intended, but I think if you want to promote your modding services there is an established 'members of the trade forum' just for that purpose.

x2

 

[gregeas]

I finally received my USB DAC1 this week. So far I've done a bit of listening in my main system, comparing the DAC1 to my Slimdevices Transporter (the Transporter is being used to send a digital signal via BNC coax to the DAC1). It's not obvious at this point that one or the other is lacking or superior, but more listening will follow this week.

I bought the USB DAC1 to be used primarily at work and also in my other systems when I want to add a high-end PC source. I love the fact that I can carry the DAC1 and my XPS 1210 laptop (with 300 lossless albums) in my small laptop bag.

I do have two questions for Elias.

First, the Wiki entry for setting up iTunes for XP states that "Word-lengths of longer then 16-bit are truncated to 16-bit, rather then being dithered.
Truncation will cause severe distortion." Does this mean that iTunes should be avoided in preference of another playback program? If so, which is optimum for XP? I have Jriver Media Center installed, but it doesn't look like this has been tested by Benchmark.

Second, and this may be stupid, but does the BNC cable twist into place onto the jack? This is how BNCs work for video, but I can't get my new cable twist into place, and I don't want to force things.

Looking forward to spending more time with the DAC1!

PS: The Transporter was good enough to force a quick sale of my $2.5k Arcam CD player. In fact, for reasons I can't explain, the Transporter crushed the Arcam in my system (which is otherwise all Arcam components, with PMC speakers).

 

[gregeas]

EDIT: A restart solved the iTunes issue below, but I'm still curious if I'm sacrificing quality due to the bit truncation.

* * * * * * *

I have another question. For some reason iTunes won't output to the DAC1 and instead plays through my laptop speakers. This is despite setting the output to the DAC1 in the Sounds and Audio Devices control panel and in iTunes. Weird.

Anyhow, I'm trying to use Jriver Media Center as a temp solution before I try Foobar, which will take some time to set up.

My question is what output mode I should use: Media Center gives options for Asio, Wave Out, and Direct Sound. Asio is what I have used in the past to get (alleged) bit-perfect output. I am aware that I should set output depth to 24 bits and turn off all audio processing. Anything else?

 

[Lord Chaos]

I'm not Elias

but I do have two DAC1s. The BNC connector is twist-on bayonet. It helps if you push to compress the disc spring that's inside the BNC connector while you twist it onto the bayonet mount.

The word-length issue is discussed earlier in this thread. It's not a problem unless you use Itunes' Equalizer, or set the volume for less than 100%. Windows, according to Elias, has a better volume control algorithm so you can use it as you want.