CD - P jitter Q's

Joules · 7th February 2006, 11:34 PM

I know very little about digital audio, and there's some thing I don't understand.
So... In a standard CD player, digital info is read from a CD. Isn't it then read to a memery buffer and reclocked and then sent off to the dac, thereby eliminating machnicaly induced jitter?

gmarsh · 8th February 2006, 01:36 PM

This is true - data streams off the CD into a buffer where it is decoded, error-corrected and output as PCM audio using a crystal clock. A servo loop maintains the speed of the CD so that the buffer doesn't over or underflow.

So theoretically, no mechanical effects (other than those causing buffer over/underflows) should introduce jitter on the output of the CD player. Though practically, electrical noise generated by the spindle motor could find its way through the power supply and affect the clock device, and mechanical "shake" could affect the clock device too.

And there's probably people that believe that weird quantum effects caused by the spinning CD can alter space/time within the CD player and create jitter.

Joules · 8th February 2006, 03:22 PM

Thanks gmarsh
This is what I thought. Then why do we worry about mechanicaly induced jitter? - Now let me say, I have added mass and damping to CD
transports and have heared, under no uncertain terms, huge improvements. So now I am realy comfused ????

davidsrsb · 8th February 2006, 03:29 PM

Typical commercial CD players have a poor quality clock, usually just a gate oscillator integrated into the decoder chip. This means that electrical noise from the servo and motor and mechanical vibration all cause fairly high levels of jitter.

The clock distribution from this oscillator to the DACs is crude too, picking up jitter on the way.

Jocko Homo · 8th February 2006, 03:39 PM

There used to be a doo-hickey called an Iso Drive. Worked in Philips CDPs. Basically, a big disc you put on top of the CD itself.

The sonic effects were the same as lowering jitter any other place.

Mass = inertia, so that might be how it did what it did. Problem was.........Philips started using cheaper and cheaper spindle motors, and they were not stout enough to spin with it on.

(I have 3 of them...........4 if you count the warped one.)

Jocko

Joules · 8th February 2006, 04:06 PM

sooo.. You could make the ultimate cdp from even a cheep player just by adding a good clock - Right? And the quality of the transport wouldn't matter ?

rfbrw · 8th February 2006, 04:35 PM

Quote:

Originally posted by gmarsh

And there's probably people that believe that weird quantum effects caused by the spinning CD can alter space/time within the CD player and create jitter.

Such effects would also explain the presence of a number of forum members who seem to be from another universe.

danb1974 · 8th February 2006, 07:03 PM

The transport doesn't matter as long as there are no uncorrected errors.

A good clock, dac and analog stage is all you need. Or an external jitter-insensitive dac.

davidsrsb · 9th February 2006, 02:03 AM

Wolfson have published an excellent white paper on jitter here:
http://www.wolfsonmicro.com/uploads/...erformance.pdf

It is interesting that jitter spoils the effects of digital filtering on out of band signals

7th February 2006, 11:34 PM	#1
Joules diyAudio Member Join Date: Nov 2004 Location: Manassas virginia usa	CD - P jitter Q's I know very little about digital audio, and there's some thing I don't understand. So... In a standard CD player, digital info is read from a CD. Isn't it then read to a memery buffer and reclocked and then sent off to the dac, thereby eliminating machnicaly induced jitter?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
generate jitter test signal and measurign jitter using AP2?	BJAMR	Digital Source	11	1st November 2010 02:07 PM
Jitter in DAC.	ghee0	Digital Source	44	3rd June 2010 09:25 AM
Data Jitter Versus Electrical Jitter?	300_baud	Digital Source	8	16th July 2004 07:59 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

8th February 2006, 01:36 PM	#2
gmarsh diyAudio Member Join Date: Apr 2004 Location: Halifax, NS, Canada	This is true - data streams off the CD into a buffer where it is decoded, error-corrected and output as PCM audio using a crystal clock. A servo loop maintains the speed of the CD so that the buffer doesn't over or underflow. So theoretically, no mechanical effects (other than those causing buffer over/underflows) should introduce jitter on the output of the CD player. Though practically, electrical noise generated by the spindle motor could find its way through the power supply and affect the clock device, and mechanical "shake" could affect the clock device too. And there's probably people that believe that weird quantum effects caused by the spinning CD can alter space/time within the CD player and create jitter.

8th February 2006, 03:22 PM	#3
Joules diyAudio Member Join Date: Nov 2004 Location: Manassas virginia usa	Thanks gmarsh This is what I thought. Then why do we worry about mechanicaly induced jitter? - Now let me say, I have added mass and damping to CD transports and have heared, under no uncertain terms, huge improvements. So now I am realy comfused ????

8th February 2006, 03:29 PM	#4
davidsrsb diyAudio Member Join Date: Dec 2005 Location: Kuala Lumpur	Typical commercial CD players have a poor quality clock, usually just a gate oscillator integrated into the decoder chip. This means that electrical noise from the servo and motor and mechanical vibration all cause fairly high levels of jitter. The clock distribution from this oscillator to the DACs is crude too, picking up jitter on the way.

8th February 2006, 03:39 PM	#5
Jocko Homo Banned Join Date: Feb 2002 Location: As far from the NOSsers as possible	There used to be a doo-hickey called an Iso Drive. Worked in Philips CDPs. Basically, a big disc you put on top of the CD itself. The sonic effects were the same as lowering jitter any other place. Mass = inertia, so that might be how it did what it did. Problem was.........Philips started using cheaper and cheaper spindle motors, and they were not stout enough to spin with it on. (I have 3 of them...........4 if you count the warped one.) Jocko

8th February 2006, 04:06 PM	#6
Joules diyAudio Member Join Date: Nov 2004 Location: Manassas virginia usa	sooo.. You could make the ultimate cdp from even a cheep player just by adding a good clock - Right? And the quality of the transport wouldn't matter ?

8th February 2006, 07:03 PM	#8
danb1974 diyAudio Member Join Date: Jul 2005 Location: Timisoara	The transport doesn't matter as long as there are no uncorrected errors. A good clock, dac and analog stage is all you need. Or an external jitter-insensitive dac.

9th February 2006, 02:03 AM	#9
davidsrsb diyAudio Member Join Date: Dec 2005 Location: Kuala Lumpur	Wolfson have published an excellent white paper on jitter here: http://www.wolfsonmicro.com/uploads/...erformance.pdf It is interesting that jitter spoils the effects of digital filtering on out of band signals

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)