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Old 25th March 2007 | Show parent
Gear Nut
RADAR DC Audio Loop Back Test

Tonight I went to the office and did the RADAR DC File loop back test in an attempt to understand why some RADAR users are saying that when they play audio files from the hard disk of a DAW, through AES (or some other digital I/O) into RADAR and then out RADAR's converters, it sounds different than when the exact same file is played from the RADAR's hard disk out through it's converters. Is it imagined, or is it real?

The two paths are:

Path 1: Audio file played from DAW disk-> DAW Software-> DAW AES Out-> RADAR AES In-> RADAR Software-> RADAR Converters/Analog Out - RADAR is the clock.

Path 2: Audio file played from RADAR disk-> RADAR Software-> RADAR Converters/Analog Out. RADAR is the clock.

Theoretically, Path 1 and Path 2 should yield the same sound since, in both cases, the same clock and converters are used. However, users are insisting that there is a difference. The only way this is possible is if the audio data is changing in some way between the two paths. To try to understand where the difference is coming from I set up the following test:

First, I created a BWAV audio file containing all 1s (Hex) so that when you scroll through the contents of the file with a Hex editor, all you see is 1s, making it easy to spot differences when comparing to another file that is supposed to be identical. I've attached the created file, "DC Audio 11 Hex - Original.WAV", to this post. I opened the file with a audio editor that I downloaded called GoldWave. The waveforms below show what the file looks like in the time domain - just a block at -17.5 dB:

Zoomed in to 132 ms across the screen it looks like this:

Zoomed in a bit more to 18 ms across the screen it looks like this:

I played the audio file off the RADAR hard disk on track 1, sent it out the 2 channel AES/EBU port and then back into the AES In. During play back, I recorded the incoming signal back onto RADAR's audio hard disk on track 3 using Autopunch to make sure the two files were the same length. I've also attached the resulting recorded file, "DC Audio 11 Hex - Recorded through AES.WAV", to this post. Then I scrolled through both files and up loaded a screen shot of the two files being compared side by side. The files are identical and are shown below:

This test can be performed on any recorder. Earlier I posted the results of a Sonar test sent to me by a user, Stormkloud, and the comparison of the files showed that they were significantly different. I've re-posted it case any of you missed it:

These files look significantly different, so I'm wondering if the test was done right. If the test was performed correctly, then it would appear that the DAW is modifying audio data, just by playing back the file.
Attached Files

DC Audio 11 Hex - Original.WAV (4.88 MB, 107 views)

DC Audio 11 Hex - Recorded Through AES.WAV (4.88 MB, 100 views)