OK...this one's a bit bizarre, but I wanted to know if other 990 users out there are experiencing a similar issue. I ahve a ticket in for this, but it sounds like the Outlaws are pretty busy with new product development, and as such, I'm trying to determine if the fault is in my unit (specifically) or if this is a global issue that was somehow missed.

I should mention that I first noticed this when the unit was new, but at the time, only on the FM tuner - I had no analog sources connected at the time - and I don't listen to FM much, so this kind of fell off of my radar.

Note: what I am about to describe does NOT happen with digital sources - only analog ones.

OK, so one day while playing a recording of waves on a beach (which, at times, approximates white noise) from my Turtle Beach Audiotron, when I switched to the "CD" input (which gets the analog signals from the Audiotron), the character immediately changed. I have the analog inputs connected so that this signal can be routed to Zone 2, which then feeds the rest of my house.

Anyway...with the analog input selected, I noticed a distinct comb-filtering effect. Before anyone out there attributes this to low bit-rate mp3 artifacts, keep in mind that my music library is in .wav format (uncompressed), not any lossy format. Also, that this was not present when listening to the digital inputs told me that it was not the source file.

Back to the issue...when I switched to the optical input, the comb filtering stopped. I switched back to the analog input, and the comb-filtering returned. Mind you...this is MUCH harder to hear with music, but the 'ocean waves' file, due to its nature, has properties that approximate white noise (at times); white noise is one of the most useful stimuli out there to use for diagnostics.

While still listening to the analog signal, I started to switch between stereo, bypass, and upsample modes. Though the character (timbre) of the comb filtering changed as I cycled through the modes, it was always there. The one exception was one of the Dolbly modes - in this mode (or maybe it was two of the Dolby modes...I have to check my notes) it sounded normal. However, when using the digital input, cycling through the modes did not produce the comb-filtering effect. I even went as far as to remove the Audiotron's optical 'send' (leaving only the analog connections in place) to see if there was some odd crosstalk between the CD input (analog) and the optical input. Regardless of whether the optical feed was present, this problem continued.

I then listened to the Audiotron's analog outputs using headphones (the headphone jack on that unit is normaled to the analog outputs) knowing that if the problem was in the Audiotron, then I would hear it over the headphones. Result? No comb-filtering whatsoever.

Hmmm...

So to help me diagnose this, I cooked up a white noise wav file and loaded it onto my iRiver portable player (which supports mp3, wav, FLAC, etc) and connected its analog outputs to the CD inputs. Sure enough, the same comb filtering that I hear while playing back the wav file from the Audiotron was present.

I then tried the other analog inputs (AUX, TAPE) and the very same thing resulted.

To document what was going on, I then connected a four-channel R-44 recorder to the outputs of the 990 - the first pair were the "front" signals and the 2nd pair were the "Zone 2" signals. I connected the analog outputs of the mp3 player (playing the white noise file) to the CD inputs and recorded about 30s of each mode (stereo, bypass, upsample etc). In this way, I would have a recording of each mode and what was going in in Zone 2 at the same time.

Result: All of the Zone 2 recordings are white noise and nothing but white noise - absolutely no comb-filtering effect was noted or showed up in the power spectra. However...what I had heard (from my speakers, which get the FRONT feeds) in all of the modes that I originally listened to while noticing this were (not surprisingly) present in the recordings.

Again...Zone 2 (which had been assigned to CD) was always absent this comb filtering.

The spectra reveal, conclusively that comb-filtering is taking place. Again, it's not all that perceptible with music, and you have to quickly switch between analog and digital feeds to have a good comparison, but the fact is, it is there - white noise is a great diagnostic tool.

How do you get comb-filtering? Well, it comes from 'delay and add' in a digital system. That is, if you look at the frequency spacing of the peaks in a classic comb-filter, the delay is proportional to that frequency's inverse. That is, a 1 ms delay and add, will produce maxima every 1000 Hz; a 2ms delay will produce maxima every 500 Hz...and a 10 ms delay will produce maxima every 100 Hz.

Incidentally, I was able to recreate this comb-filtering effect as a simulation (using the amount of delay noted in bypass, stereo, and upsample modes), and the resulting files have almost identical spectra as the files created from the 990's front outputs. Moreover, if you listen to the simulation as compared to what is coming out of the 990, you really cannot tell a difference. based on this, I am sure there is a 'delay and add' issue going on in my unit.

So...here are my questions:

a) when listening to FM (especially if off-station when the level of hiss is high), if you cycle the modes (stereo, bypass, upsample...etc) does the quality (the timbre) change?

b) has anyone tried sending white noise to their 990 via the analog inputs and cycling through the modes to see if they notice it?

Again, it's harder to hear with program material, but unmistakable when using white noise as a stimulus.

Comments?

Helluva first post!! Let us know what Outlaw says? My 990 is not out of warranty yet. Did you get a chance to check the phono input? Since zone 2 doesn't have an issue, it is likely a processing issue. Do you use your 990 for HT and do you have your lip sync delay engaged? I don't think the lip sync is what you are hearing but the circuit might be an issue.

@Xenon:

Thanks for the kudos. It's actually not my first post - I used to post as "old_school", but it has been some time since I participated in the forum, and consequently, I could not remember my password...and deleting the profile would have deleted my prior posts.

Anyway...

I'm convinced it is processor related, specifically for the reason you cited about zone 2 (i.e. on the analog buss and that it has no interaction with the parallel digital processing).

Another reason that I am convinced is that I was able to synthesize three wav files, starting with white noise, and applying the delay and add approach using the time increment derived from the distance between maxima in the spectra - and what I saw makes perfect sense that this is going on and is a function of the sample rate invoked (i.e. stereo, bypass, upsample). Again, if you play the files (and I have them for anyone who might be interested in hearing them as well as the plots of their power spectra) you really cannot distinguish my re-creation (via synthesis) from the original files. Spectrally, they ARE different in the sense that while the spacing of the peaks match exactly, in the synthesized versions, the valleys are sharper; I attribute this to the fact that my synthesis was dleay and add, and therefore, does not take into account the mathematical impulse response of the 990. I could probably derive that though, and my guess is that were I to convolve the impulse response obtained from the three modes (stereo, bypass, upsample) with white noise, that their spectra would be an exact match to the data that show the issue.

I have not tried the Phono input, but I would bet the farm that in zone 2, it would not have the comb-filtering, but in the front, it would be there.

My hypothesis is this:

There is (obviously) an analog buss. Signals are taken from the analog buss and fed to an A/D, and then on to the digital engine - so far, so good. Within the digital engine the signal is manipulated however the user chooses. This signal is then fed to a D/A. Again, so far...so good.

However, my guess is that the output of the D/A feeds a summing junction on the analog buss, right before the output. This would completely explain the delay and add issue, because there is latency (processing or 'transit' time) in the digital path. Moreover, the amount of the latency is proportional to the sample rate - which explains why the timbre of the comb filtering shifts (in particular) in the upsample mode.

I suspect that there is a switch at the summing junction (whether an electronic switch or a relay) that is supposed to disconnect the output of the D/A before the summing junction, thereby ensuring that there really is a true bypass. After all, bypass is supposed to bypass all processing circuitry, but the fact that comb filtering happens even in the bypass mode tells me that analog and digital signals are being summed. What puzzles me is why the dolby mode(s) don't exhibit the comb-filtering, but frankly, I don't really know a lot about those codecs.

Anyway, it seems to me that the way the analog and digital traffic should be managed is thusly:

*Assuming* the analog signal is always fed to the digital section, there should be a two-position 'switch' after the D/A; in one position, the output of the D/A is fed to the summing junction (digital modes). In this switch position, a second switch is in a complementary state. That is, when any digital mode is selected (i.e all but BYPASS) the analog signal is disconnected from the summing junction.

Using this architecture, it would be impossible for 'delay and add' to occur, because the analog signal (from which the digital signal is derived...and with latency) could not be summed with the delayed digital signal.

Conversely, in bypass mode, the two switches each have to change state to their respective complementary states (complementary to the states used in digital modes). That is, in bypass, the signal that is rattling around in the digital section is effectively disconnected from the summing junction, and the only thing that the junction can add is the analog signal on the analog buss to "zero" coming from the digital section.

As an electrical engineer, I know that when digital systems are created, truth tables are employed to determine the combinational or sequential logic. However, sometimes, there are conditions called 'don't care' that stem from the fact that a particular combination of inputs will never arise, or if they do, the designer 'doesn't care' what stse the device is in - this is done to keep costs low, but in 'critical' systems (i.e. those in which human life hangs in the balance) don't care's are very carefully managed - don't care's become 'you better know what this will do when these particular inputs show up and you had better act accordingly'.

So, if there are in fact these switches present (i.e. my guess at the architecture is correct) and if they are addressable electronically, then my guess is that someone missed how the don't care's were assigned. That's actually my hope, because this would seem to be something easily remedied in a firmware update (i.e. proper management of the switches, if indeed this is how the architecture works...which again...I don't know...I'm just looking at what the data are telling me, as well as the results of my own experiments to get a handle on this).

However, if the analog and digital (post D/A) streams are always summed (i.e. none of these 'switches' or routing provisions exist), then it would be impossible - mathematically and physically - to prevent a delay and add issue from occurring; this would be a much bigger problem for those who wish to listen to analog inputs in the main zone. Again, if all you care about is zone 2, then this is not an issue...

...but...there are analog inputs...and...

...there is a 'bypass' mode which is supposed to be purely analog...but how can comb filtering happen if the bypass is really working?

All of the math point to a delay and add scenario. Maybe later I can post the delay times that I calculated.

One other thing...I am not grousing about the delay - I could care less about that, because merely delaying a signal will not produce comb-filtering. However, delaying it and adding it will do this. In fact, this is the heart of flanging (Think of the bridge in the Doobie Brothers' "Listen to the Music" or the Small Faces' "Itchykoo Park" and you'll know what I am talking about). The only difference between comb filtering and flanging is that the amount of delay time is variable, and thus, as one sweeps the delay time, you get that inside-out sound, but it changes in timbre as the flanging goes on; stop varying the delay and you get a comb filter once more.

I do use it for HT; I take the optical from the DVR and to be honest, I have never had to experiemnt with the lip-sync delay. However, I don't see this as associated to the problem...but I could be wrong. Still, it seems unlikely, because there is no delay on the analog buss - all the delay that takes place does so in the A/D --> PROCESSING --> D/A path...because discrete sampling is going on.

Phew...time for a drink. Again, if anyone out there wants to hear / see the files in question, let me know...but a simple experiment would be to set the tuner to a non-station (to generate something close to white noise) and then cycle the modes (stereo, bypass, upsample, dolby, dolby vs, dts etc) and listen carefully to see if your 990 exhibits the same issue.

This could simply be a defect in my unit (bummer for me, but at least otehrs don't have to deal with it)...or it could be an oversight (back to the overlooked don't care's thing...) ...or it could be a design ovesight for which there is no fix. Again, I have no '2nd 990' lying around against which I could compare this particular 990, so I don't know what the answer is (obviously...or I would not have posted this).

Thanks...

Mark

Though I try to avoid Wiki...here's one link about comb-filtering that may prove useful:

http://en.wikipedia.org/wiki/Comb_filter

And here's another:

http://www.google.com/imgres?imgurl=http://www.music.mcgill.ca/~gary/307/week2/images/backward-comb.jpg&imgrefurl=http://www.music.mcgill.ca/~gary/307/week2/delaylines.html&usg=__ZRNNlt70fyz_nsQ3glr8c3hczvw=&h=291&w=713&sz=49&hl=en&start=20&zoom=1&tbnid=4EJaP1DQZRuOtM:&tbnh=57&tbnw=140&ei=dXl7T83PLInftgfBh4G4CA&prev=/search%3Fq%3Dcomb%2Bfiltering%2Bdelay%26um%3D1%26hl%3Den%26sa%3DN%26gbv%3D2%26rlz%3D1R2ADSA_enUS355%26tbm%3Disch&um=1&itbs=1

I hope the links work...

I will be near my 990 in about two weeks and will try to duplicate your effort with one of the many radio stations I don't get. You say you noticed this right off the bat when you got your 990? I listen to analog all the time via radio and phone and never experienced an issue but my hearing isn't the best. With the white noise what should I hear if the problem exists? Some sort of frequency mod that changes when I change the sample rate? What setup do you have for the following statement:

I have the analog inputs connected so that this signal can be routed to Zone 2,

Is it possible that the input wiring is causing the add signal to be generated and summed with the delay through the digital path? I am only guessing but there is probably a way to crosstie the digital signal to the analog buss with some creative wiring in the back. Could it be some sort of crosstalk problem?

Well, I don't know the architecture of the processor, but in essence I don't know how the analog signals could mix with the digital ones once the analog signal hits the A/D; all of the processing within that section is digital. The most likely scenario is the delay and add approach. That is, the only way that I can see crosstalk occurring is via delay and add; I can't think of a way for the analog signal to get into the digital signal (and indeed, they are two completely different domains).

It's funny because I did in fact try to remove the optical feed from the source when I first noticed it (leaving the analog connected - see thread's origin)...and soon I discovered that it wasn't he source (verified by using yet anotehr analog source). Again, per the post, I have routed the analog "CD" input to zone 2, and this is the input that I used with the stand alone analog source, bot to listen and confirm, as well as make the recordings.

If you want to know what to listen for, you could email me, and I could send you the mp3's (they are 320 kbps so they are very, very close to the original wav files). If you listen to these, you will know immediately what to listen for. I could show you the plots as well if you are interested. One thing to note - you may need to slightly adjust the voluem of the mp3's when you play them, because I didn't botehr to normalize their levels. Still, if you compare the mp3 from zone 2 to any of the ones taken from the front (stereo, bypass, upsample) the difference is immediate and unmistakable.

Thanks for being willing to check into this - I really do appreciate it.

So... have any other 990 owners tried doing this? I mean, feeding a white noise signal to an analog input, and toggling between the various modes? Incidentally, to give you an idea of what comb-filtering sounds like...if you put a cardboard tube up to your ear...you're hearing comb-filtering. Mind you, the nature of the comb filtering depends upon the length of the tube, but you get the idea.

Finally, it was easy for me to discern the difference between the sigital input version and the analog input version (of the same signal). Lastly, if you do not have a white noise .wav file (or high bit-rate mp3), you could use the sound of ocean waves as a surrogate test signal, or email me and I'll provide a white noise signal to you for use (though there are test CDs out there that feature white noise...I'm sure).

Thanks,

Mark

So, I went back and looked at the data...

Though I do not have them all handy, I have them...but for the STEREO and UPSAMPLE modes, I have the values. Here they are:

UPSAMPLE Mode:

Delay: 9.3 ms
Comb Spacing: 107 Hz

STEREO Mode:

Delay 8.0 ms
Comb Spacing: 125 Hz

Again, I am not simply saying that there is a 9.3, an 8.0 or whetever latency - I would be OK with that. What I am saying is that a delay and add scenario is taking place, which is turning the analog signals (at least in the main speakers) into a comb-filtered version of what is coming into the analog inputs; again, zone 2 is not affected by this issue.

How do I know the delay times? I looked at the spectra, and from them, I derived the delay time that is consistent with the peaks in the spectra. That is, in the UPSAMPLE mode, the spectra exhibits regular peaks (about 10-12 dB delta...not insignificant) every 107 Hz and in the STEREO mode, these peaks occur every 125 Hz.

For the math minded you will note that the reciprocal of the delay time indicated you get the spacing of the peaks. That is, take the reciproc al of 0.008 (which is 8 ms) and you get a frequency of 125 Hz - and this is indeed (pretty much exactly) the spacing of the peaks in the spectrum for the stereo mode. For the upsample mode, the peaks are a bit more tightly sapce due to the longer delay time of 9.3 ms (that is 0.0093); take its reciprocal and you get 107 Hz, and this is indeed where the peaks are spaced.

So please, would someone out there play a white noise file into the analog inputs of their 990 and see if you notice the comb-filtering? Again, you will notice this as you change modes (STEREO, BYPASS, UPSAMPLE etc).

Thanks,

Mark