Re: Hiss Effect
Re: Hiss Effect
- Subject: Re: Hiss Effect
- From: Brian Willoughby <email@hidden>
- Date: Wed, 18 May 2011 20:46:39 -0700
On May 18, 2011, at 16:33, Admiral Quality wrote:
There's going to be effects from the stereo encoding as well. As a
vinyl record uses an M-S scheme, you'll get some clicks/pops in the
center (the M signal, lateral movement of the needle), but you'll also
get others that come out phase inverted as they're only in the -S
signal (vertical movement of the needle).
Vinyl records do not use an M-S scheme. There is only one needle,
and it can only be in one position at a time, but there are two
degrees of freedom of movement. Stereo records encode left and right
at 45 degrees each, such that the two are ninety degrees apart. What
you describe is merely a side-effect of having one needle track two
signals - you can think of it like mid-side, but it's still literally
left-right signals going into the cutting lathe. Wikipedia says some
early stereo lathes were cobbled together by using a lateral cutter
and a horizontal cutter, and those hacks were M-S, but the common
design is L-R.
Regardless of the fact that vinyl is not cut M-S, you still have to
consider the ramifications of a single needle tracking two signals.
Vertical movement will affect both channels in phase, and lateral
movement will affect both channels in opposite phase.
If you want to closely emulate vinyl deformations, you might consider
a polar coordinate system. Then you would calculate individual pops
by combining an intensity with an angle. The exact angle of the
deflection would determine how much each channel is affected, and
whether the noise is perfectly in phase, perfectly opposite in
polarity, or somewhere in between. Each pop, assuming it results
from a foreign particle on the record surface, would have an
independent angle of deflection depending upon its position in the
groove.
On Wed, May 18, 2011 at 7:29 PM, <email@hidden> wrote:
I was in a totally misconception of the RIAA curve, I thought it
was applied
to the record to cut the bass it can't handle and not to the playback.
Thanx for the explanation.
There are two parts to RIAA. There is the encoder that is only used
in the cutting lathe electronics or sometimes printed onto the stereo
master tape, and then there is the decoder that it found in every
home or discotheque. The encoder cuts bass to avoid extreme
excursions while boosting highs to keep them above the noise floor,
because each octave contains twice as much energy as the previous,
causing the noise to sound like it has more high frequencies. The
decoder boosts the bass coming off of the record to compensate for
the reduction during cutting, while cutting the highs to reverse the
encoder. Since the noise is introduced in the encoded analog media,
the RIAA decoder will cut most of the highs out of any surface noise,
but unfortunately will boost any low frequency noise signals.
If you were to process synthesized surface noise with an RIAA
decoder, it should produce the rumble-heavy but not-very-hissy sound
of vinyl. Just mix that with the standard audio and you'll be good.
There are a few minor details to add.
A) Some turntables have a rumble filter to cut the bass, but those
remove bass from the original material along with the wow and rumble
of the media. Thus, you might not boost your synthesized noise below
20 Hz or 10 Hz, depending upon which playback system you want to
emulate.
B) Cutting lathes could burn out with the actual high-frequency
boosts called for in the RIAA encoder, where the boost continues to
increase at constant number of dB per octave as the frequency gets
higher and higher. To protect the lathes, some cutter electronics
put a low pass with a corner frequency of 50 kHz. A standard RIAA
decoder would thus not restore frequencies near or above 50 kHz that
might have been present on the original analog tapes. An improved
RIAA decoder will mirror the 50 kHz low pass by allowing more
ultrasonic frequencies on output, thus restoring the original analog
signal. This effect is almost entirely irrelevant for an iOS device
with digital audio at 44.1 kHz or 48 kHz maximum. I say almost
entirely irrelevant because the EQ curves below 24 kHz are still
slightly affected by the difference.
Brian Willoughby
Sound Consulting
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Coreaudio-api mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden