Audio out of phase?

[Richard Amirault]

I have a AIW Radeon 7500 and I've just noticed a problem with the sound
while watching and recording TV.

The stereo audio channels seem to be "out of phase" with each other. That
is, the same signal on both channels (vocal dialog for instance) will cancel
out, while background music (since it is different on each channel) comes
thru.

Some of the channels are more affected than others, and of those that are
affected the left/right slider in Windows Audio control panel can make the
problem worse or better depending on the position of the slider. For
instance the two PBS channels I receive are terrible with the slider dead
center. While most of the other channels sound OK dead center .. but if I
move the slider off to the right a bit I can get the same effect (but maybe
not quite as bad as the PBS channels)

I've tested this by recording stereo audio off the TV tuner and during the
recording I move the slider from the "null" position, to full left and full
right. While the waveform shows good signal on both channels, the playback
fo the file gives the same problem .. speech is cancelled out, but moving
the slider to one side or the other give perfect audio for that side.

MY QUESTION: Is this a problem with my computer set-up? OR is it a problem
with my cable company. NOTE: watching the same channels with a 'regular'
stereo tv does not show the problem at all. I've been using this card for
ages and this is the first time I've noticed this problem. I haven't done
any physical changes to the computer lately except plug in a new video
monitor.

If you want to listen to the short sample file:
http://n1jdu.org/temp/Bad_Audio_AIW_7500.mp3 (about 75k) The clip starts
off with the problem, then I quickly shift the slider to the right, then to
the left, then back to center.

 

[stratus46]

I have a AIW Radeon 7500 and I've just noticed a problem with the sound
while watching and recording TV.

The stereo audio channels seem to be "out of phase" with each other. That
is, the same signal on both channels (vocal dialog for instance) will cancel
out, while background music (since it is different on each channel) comes
thru.

Some of the channels are more affected than others, and of those that are
affected the left/right slider in Windows Audio control panel can make the
problem worse or better depending on the position of the slider. For
instance the two PBS channels I receive are terrible with the slider dead
center. While most of the other channels sound OK dead center .. but if I
move the slider off to the right a bit I can get the same effect (but maybe
not quite as bad as the PBS channels)

I've tested this by recording stereo audio off the TV tuner and during the
recording I move the slider from the "null" position, to full left and full
right. While the waveform shows good signal on both channels, the playback
fo the file gives the same problem .. speech is cancelled out, but moving
the slider to one side or the other give perfect audio for that side.

MY QUESTION: Is this a problem with my computer set-up? OR is it a problem
with my cable company. NOTE: watching the same channels with a 'regular'
stereo tv does not show the problem at all. I've been using this card for
ages and this is the first time I've noticed this problem. I haven't done
any physical changes to the computer lately except plug in a new video
monitor.

If you want to listen to the short sample file:
http://n1jdu.org/temp/Bad_Audio_AIW_7500.mp3 (about 75k) The clip starts
off with the problem, then I quickly shift the slider to the right, then to
the left, then back to center.

I loaded your file into Adobe Audition 1.5 and looked at the 'phase
analysis'. There is nothing unusual with the phasing of the channels
though with your slider centered, the right channel is about 3dB hotter
than the left. Any chance the sound system connected to the computer
has a phase turnover? The most common fault is one speaker wire
reversed hot/ground.

Your description sounds like the ground wires of the 2 speakers are
connected to each other but not the ground terminal. Mono programming
would cancel out but with a stereo source, you'd hear only the
differences between the channels.

GG

 

[Richard Amirault]

<stratus46 wrote ...

I loaded your file into Adobe Audition 1.5 and looked at the 'phase
analysis'. There is nothing unusual with the phasing of the channels
though with your slider centered, the right channel is about 3dB hotter
than the left. Any chance the sound system connected to the computer
has a phase turnover? The most common fault is one speaker wire
reversed hot/ground.

Your description sounds like the ground wires of the 2 speakers are
connected to each other but not the ground terminal. Mono programming
would cancel out but with a stereo source, you'd hear only the
differences between the channels.

I don't see how the speaker connections would affect the recording I made.
If the problem were only after the sound leaves the computer, yes, but the
recording was made by the same computer I was watching the TV show on .. in
real time ... and reflects accurately what I hear from those speakers.

If, somehow, the phase was reversed at the cable company (or higher in the
chain) would your analysis still see it, or does that program only see phase
reversals closer to the end useage?

As I've said, I've had this card for ages, and never noticed this problem
before .. nothing has changed on the computer .. except about a month ago I
had to re-install all the ATI software .. and actually after that things
seemed to work better than before.

 

[T Shadow]

<stratus46 wrote ...
As I've said, I've had this card for ages, and never noticed this problem
before .. nothing has changed on the computer .. except about a month ago I
had to re-install all the ATI software .. and actually after that things
seemed to work better than before.

Did you reinstall the sound card drivers? Your ATI installation may have not
been completely successful or it may have hosed the sound card installation.
My sound card drivers get hosed about 20% of the time.

 

[Richard Amirault]

stratus46 wrote ...

I loaded your file into Adobe Audition 1.5 and looked at the 'phase
analysis'. There is nothing unusual with the phasing of the channels
though with your slider centered, the right channel is about 3dB hotter
than the left.

(snip)

By the way .. if you still have that file .. try muting the right channel
and you'll hear the left channel *perfectly* all the way thru to the end,
then reverse it and mute the left channel and you'll hear the right channel
just as well. I'm not an audio tech .. but this says to me that it's a
phase reversal problem.

Now .. WHERE is this happening? That's the question.

As far a one side being hotter I'm not absolutly certain the slider was
perfectly centered.

Richard Amirault N1JDU Boston,
MA, USA
n1jdu.org "Go Fly A Kite"

 

[stratus46]

stratus46 wrote ...
(snip)

By the way .. if you still have that file .. try muting the right channel
and you'll hear the left channel *perfectly* all the way thru to the end,
then reverse it and mute the left channel and you'll hear the right channel
just as well. I'm not an audio tech .. but this says to me that it's a
phase reversal problem.

Now .. WHERE is this happening? That's the question.

As far a one side being hotter I'm not absolutly certain the slider was
perfectly centered.

Richard Amirault N1JDU Boston,
MA, USA
n1jdu.org "Go Fly A Kite"

I did still have the file and did as you suggested. When muting the
right channel and playing only the left, at 7 seconds and 2 frames, the
audio fades down about 20dB and returns to normal at 10 seconds, 7
frames. I just loaded it into another computer with the same software
and it does the same thing complete with level changes and the 3 dB
imbalance. Both computers have on-board sound and are using the
mother-board supplied drivers. One is an ECS and the other a Gigabyte.
Do you have some sort of special drivers to process Dolby 5.1 or
similar?

GG