REW setup with audio through HDMI - Page 3 - Home Theater Forum and Systems -

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post #21 of 25 Old 02-20-10, 04:46 PM
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Re: REW setup with audio through HDMI

Glad to be of help. If your SC has the ability to run in quad stereo mode or surround modes, then using a digital connection can also allow testing of the other speakers easily, and again, isolating speakers is usually a simple matter of adjusting the settings on the sound card. If nothing else, using a digital connection, assuming it works for you, does open you up to a fair few options.
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post #22 of 25 Old 02-20-10, 09:15 PM
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Re: REW setup with audio through HDMI

Greg, Dan,,

Let's start with the mathematical model I used to look at what happens when one drives the receiver for testing through a digital audio connection instead of the normal analog connection.

Let i(f) be the input freq resp of the soundcard,
o(f) be the output freq resp of the soundcard,
a(f) be the freq resp of the analog inputs to the receiver,
and X(f) be the performance of the rest of the system under test.
From here on out, I'll leave off the (f) notation, but remember these are all functions.

Case 1 (Normal):
The soundcard calibration file represents (o + i).
Running the test signal through the analog inputs, REW sees (o + a + X + i).
After subtracting the soundcard file, one measures (a + X), i.e., variation in the input ADC process is present, plus all the variation in the receiver's processing including its output DAC stage.

Case 2 (SPDIF/HDMI):
Let's suppose we use the standard soundcard file with a digital connection.
Running the test signal through the digital input, REW sees (X + i).
Subtracting the soundcard file, one measures (-o + X). Variation in the soundcard's output DAC process now appears in the measurement, in place of the analog input variation, even though the soundcard's DAC is not actually being used.

Case 3 (modified soundcard calibration):
Suppose we build on the idea of comparing the full range measurement of the main preout with analog input to the measurement with digital input.
If we drive the preout with analog input, we see (o + a + X + i). When we drive the preout with the digital input, we see (X + i). If one calculated the difference between the two curves, one has (o + a). If we apply this to the normal soundcard file (o + i), one could create a soundcard file representing (i - a).

Now look what happens when we use this soundcard calibration file with a measurement through the digital input. REW sees (X + i), subtracts the soundcard (i - a), and the resulting measure should show (a + X). In other words, one should be able to construct a soundcard calibration file that would yield exactly identical measurements with the SPDIF/HDMI input as is measured using the analog input. Of course, this does not eliminate the receiver's analog input as an element in the measured results, but the digital input may be more convenient and may introduce less noise into the measurement process.

So let's see what happens when one looks at this in the real world.

The first issue I discovered is that, although the sampling rate may not matter much when using the analog inputs, it matters a lot when using a digital audio connection to drive the receiver. In the following graph, the red line is what happens when I run a sweep using the S/PDIF connection and measuring the output on the left front preout. The green, with all the oscillations, is the identical measure except running two sweeps instead of one. As multiplying the number of sweeps should just give us more measures to average to give a smoother line, this is really weird. The light blue line shows the same measurement using the analog RCA connection to the receiver, single sweep, no smoothing, to show that it is indeed noisier than digital. This problem goes away when I take the measurements using a 44kHz sampling rate instead of 48kHz.
REW setup with audio through HDMI-100220.spdif48ktwosweeps.jpg

After changing the sampling rate to 44kHz, I thought I would see if I could eliminate some of the noise from the test by running my laptop off battery instead of line power. The unexpected result is that running off battery changes the measured response of the system, as if being on battery power changes the behavior of the soundcard. In the following graph, both measures use the digital audio connection to the receiver, measuring the output at the preout. Red is the result when the laptop is on battery, blue on the power line.
REW setup with audio through HDMI-100220.spdif44k2sweepsbattery.jpg
The good news, in my case, is that the difference is only 2dB and only above 10kHz.

Now let's compare the analog measurement to the digital measurement. In the following graph, the flat gray line is a sweep of a simple loopback not going through the receiver, the green line is the preout when the analog RCA input is driven, the red line shows the same measurement when digital audio is used to drive the receiver.
REW setup with audio through HDMI-100220.spdifvsanalog44k.jpg
This resembles Sam's measurement in the link I cited earlier. Using the normal soundcard cal file, while driving the receiver using digital audio, means the resulting measurement subtracts out the impact of the soundcard's DAC even when not going through the DAC. Note that the differences appear large because of the expanded vertical scale, 2dB per gridline.

Finally, I tried my case 3 above. I generated a cal file looping through the receiver with the analog input, another cal file looping through the receiver with the digital audio input, took the difference of these and applied that to the normal loopback cal file. Using this new adjusted soundcard cal file, I again measured a sweep driving the receiver with its digital input. In the following graph, the gray and green lines are unchanged from above. The blue line is the new measured result with the adjusted cal file.
REW setup with audio through HDMI-100220.spdifadjustedcalfile.jpg
As you can see, the measured blue graph using the digital input now matches the normal green curve using analog audio inputs. The noise comes from the process of looping back through the receiver to generate the intermediate soundcard data. I'm sure one could eliminate the noise from the data through smoothing before calculating the adjustments, ending up with a smooth soundcard cal file and smooth results, I just did not try to do this yet in Excel.

For me, I think this is a lot of work to go through to use the digital audio connection, to get the same results available with the normal analog audio connection. But, if done right, it would reduce the analog noise from the test process.

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post #23 of 25 Old 02-20-10, 11:29 PM Thread Starter
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Re: REW setup with audio through HDMI

My goodness. What have I started?

Extremely informative however, thank you all.

Gear: Panny 58v10, Onkyo 905, NHT SuperTwo x2, NHT SuperOne x5, Danley DTS-10 w/QSC RMX 2450, Panny BD60, PS3, Xbox360, Wii, HTPC
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post #24 of 25 Old 02-21-10, 11:09 AM
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Re: REW setup with audio through HDMI

Very interesting Bill, and thanks for that. It's possible that you may have achieved my goal, but I'm going to have to read that again when I have more time to digest it. Your basic premise isexactly what I've been toying with all along, I'm just a little concerned about the onise in your measurements. I'm not really surprised that running the laptpo off battery cleaned things up a bit... As a power supply Engineer, laptop power supplies are not exactly highly filtered devices.

I'm thinking real hard right now about redoing my kitchen... how long to I have to think about it before you come and just do it for me?


Don't worry... nothing new here, I've already made that mistake. Trust me.
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post #25 of 25 Old 02-21-10, 11:37 AM
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Re: REW setup with audio through HDMI

That would be sometime after I redo the tiles in my kitchen, bathroom, and front entry way. Which I am not even thinking about starting.
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