Re: Clip-level question
Re: Clip-level question
- Subject: Re: Clip-level question
- From: Jeff Moore <email@hidden>
- Date: Tue, 3 Jun 2008 18:21:42 -0700
Just so we're clear on terminology, the valid range for a floating
point sample is not [-1.0, +1.0] as you seem to be saying but rather
the range [-1.0, +1.0). The fact that +1.0 is a clipped value often
trips folks up with floating point samples.
On Jun 3, 2008, at 3:41 PM, Glenn Zelniker wrote:
Sorry if this has been asked before, but I can't seem to find a
satisfactory answer.
I am trying to determine the level at which a sine wave will produce
audible clipping.
First off, this is going to depend very highly on what audio device
you are using. It also depends highly on where you are setting the
volume slider and what the range of that volume slider is.
To wit, I am playing a sine wave (1 kHz @ -1 dBFS) from disk using a
small test program I wrote. I convert these samples to floats and
apply gain before sending the samples off to the output. I've found
that depending on the position of the system volume-control, the
amount of gain I can apply varies before clipping occurs. This would
seem to indicate that the system volume-control is operating pre-
DAC, no?
It seems to me that a more obvious answer is that it depends on what
the hardware volume control is set to, whether the device in question
is doing any sort of defensive DSP, etc.
But I've yet to find any kind of calibration information that
explains the peak level that a sine wave can achieve without
resulting in clipping, as a function of the position of the system
volume-control.
That's because we don't provide anything like that. There are two many
variables in the equation to nail that down with any sort of accuracy
for the general case. I can't even remember if the hardware tech notes
even include the line voltages the DACs put out.
With the system volume set to maximum, it appears that I can send
out values larger than 30 dB above what I would have thought was
full scale (relative to +/- 1.0 floats).
Again, depending on what device you are using, you could see
variations like this. In fact if the device is doing defensive DSP
(like with the built-in speaker on our hardware), you'll also see
variations that depend on the frequency of the signal.
So, can anyone tell me what the peak sine clip level is as a
function of the system volume control?
At a very fundamental level, this question is basically unanswerable
in the general case. At the very least, the variations in the
resistors, caps, and other bits of hardware in the audio device are
going to make such results meaningful only for a single combination of
CPU and audio device and then only if you actually go and measure it
electrically.
--
Jeff Moore
Core Audio
Apple
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