Vinyl "Compression" - another pointless arguement?

GW43

Well-known Member
With all the talk comparing vinyl sound quality to that of CDs, I wonder if anyone has considered the "compression" that occurs with vinyl as the needle tracks towards the centre of the disc.

At the outer edge of a disc the needle covers around 94cm of groove at each revolution (Pi x Diameter: 3.142 x 30cm). At the inner grooves, let's say 7cm out from the centre, the needle is covering only 43cm of groove. So, the information within the groove gradually diminishes as the needle tracks towards the centre. At the end of the last track it's less than half than at the start of the first.

There is also the increased lateral forces on the needle as it tracks towards the tighter turning circle.

I understand CDs spin a different speeds depending where the laser is reading from to ensure the amount of information retrieved is the same.

CDs are therefore more consistent in their information delivery!:devil:

"Consistently bad" some will cry!:D
 

chippyteaforme

Well-known Member
With all the talk comparing vinyl sound quality to that of CDs, I wonder if anyone has considered the "compression" that occurs with vinyl as the needle tracks towards the centre of the disc.

At the outer edge of a disc the needle covers around 94cm of groove at each revolution (Pi x Diameter: 3.142 x 30cm). At the inner grooves, let's say 7cm out from the centre, the needle is covering only 43cm of groove. So, the information within the groove gradually diminishes as the needle tracks towards the centre. At the end of the last track it's less than half than at the start of the first.

There is also the increased lateral forces on the needle as it tracks towards the tighter turning circle.

I understand CDs spin a different speeds depending where the laser is reading from to ensure the amount of information retrieved is the same.

CDs are therefore more consistent in their information delivery!:devil:

"Consistently bad" some will cry!:D

Not sure I quite understand - towards the centre the needle will need to go through more revolutions due to the circumference of the track becoming less, however the grooves themselves don't get more narrow, nor does the needle move faster through the tracks? So surely no 'compression' is taking place (other than the possible effects of lateral forces, and even then, I'm not sure of quite what effect they would actually have).
 

GW43

Well-known Member
The vinyl disc spins at a constant speed of 33rpm.

The needle travels further for one revolution of the disc at the outer edge (approx 94cm), compared with approx 43cm for each revolution at the inner edge of the grooves.

Therefore the needle is getting less than half of the info per unit of time on the last track as it did on the first.

I'm happy to keep this discussion going until the wedding coverage is over!
 

formbypc

Active Member
Therefore the needle is getting less than half of the info per unit of time on the last track as it did on the first.

No, that doesn't follow. It's travelling less distance, but it's recovering the info that is cut into the record. It's not glossing over bits of info on the disc due to it travelling less distance.

The cutter head travelled exactly the same, smaller distance at the middle than the edge when it cut the record. Did it only transfer some of the mastertape to the disc because of this?

If the head/stylus cuts/replays a 20kHz tone over either 94cm or 43cm at 33 rpm, then that means that at the edge, one cycle/hertz occupies 0.047mm of disc space, and 0.022mm of disc space at the inner.

It'll only be an issue if 0.022mm is beyond the tracing capabilities of the cartridge, but since we know that encoded quadrophonic LPs had carrier signals way beyond 20kHz, that can be discounted.
 
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GaryB

Distinguished Member
This together with the fact that tonearm geometry means that stylus alignment vs groove direction tends to be worse toward the end of the disc was a big subject on the days of vinyl - It was one of the main selling points of Linear Tracking Turntables. The various aspects of this that caused the degradation were generally collectively know as Inner Groove Distortion.
 

Mr Pig

Well-known Member
It's not complicated. The linear groove speed will be lower towards the centre of he record, yada yada yada. But so what? The only thing that matters is, can you hear it? Personally I think the answer is that it depends but generally, not really.

I get the theoretical advantage of linear tracking arms but the bottom line is can you hear when the stylus is not parallel to the groove? When it is between null points? I can't. A good sounding record typically sounds good all the way across the disk and a poor or dirty record will sound poor all the way across the disk.

I say it depends because I think that some cartridges and arms are worse than others when it comes to sounding variable. Generally, the 'better' quality the stylus is the more variable it will sound but relative to the vast differences in quality between records I think that tracking errors are pretty insignificant. If tracking error had a noticeable audible effect it would be obvious,we would notice it and all of the quality arms in the world would be linear trackers.
 

formbypc

Active Member
The other thing to bear in mind is that with a radial pickup arm, as most are, the makers' alignment tools are geared toward optimising the tracking across the disc, i.e. best at the middle of the disc, and falling off toward the edge or middle.

It is possible to optimise the tracking for the centre of the disc, rather than the middle, but this leads to worse error at the edge.
 

Alan Mac

Well-known Member
If the head/stylus cuts/replays a 20kHz tone over either 94cm or 43cm at 33 rpm, then that means that at the edge, one cycle/hertz occupies 0.047mm of disc space, and 0.022mm of disc space at the inner.

I think your record must be rotating at 1 revolution per second.



A Long Playing (LP) vinyl record should rotate at 33⅓ RPM

33⅓ RPM

= 33.3333 / 60 revolutions per second

= 0.555 revolutions per second


The distance travelled per revolution of the outer groove is about:

∏ x 300 mm

= 942 mm / revolution


So the linear velocity at the outer groove is:

= 942 mm / rev x 0.555 rev / second

= 522.8 mm / second


So at the outer grooves the wavelength of a 20 kHz tone is:

= (522.8 mm / second) / (20,000 cycles / second)

= 0.0261 mm / cycle

= 26.1 μm / cycle


Similarly, at the inner grooves, the wavelength of a 20 kHz tone is:

= ( 0.555 c/s x Π x 140 mm ) / (20,000 c/s)

= (244.1 mm/s ) / (20,000 c/s)

= 0.0122 mm / cycle

= 12.2 μm / cycle


Alan
 

Mr Pig

Well-known Member
pig%20suicide.JPG
 

steveledzep

Active Member
I have just split my sides laughing. I called the wife in to have a look, she is rolling about on the floor. Do you think the Audio Diff Maker Software (do I remember it correctly ? ) could be handy here ? Mr Pig, you entertain me so very much. Your obvious HiFi knowledge, humour and sometimes your terseness. Keep it up !!
 

formbypc

Active Member
OK, so I got the math slightly wrong. I am humbled.

The principle still holds, though - we're not 'losing half the info' because of any change in the relationship of signal to length of groove.

Agreed, Alan?
 

BlueWizard

Distinguished Member
I agree the length of the groove in unrelated to the amount of data that can be cut into it. Well, not totally unrelated, but unrelated in the context we are using here.

This represents a degree of mechanical compression, but the fidelity of the music in the inner grooves is not really change.

Keep in mind, records used to spin at 78 rpm, and they sounded fine. Their limitation was the mechanics of cutting them, not in they speed the spun at.

Now, though it is far less common, it is possible for the actual audio on a vinyl disc to be compressed. Back in the bad old days I thought it would be cool to buy some Greatest Hits of the (insert decade here) albums. They were absolute trash. To get so many hit songs into the space of one album, they were horrible compressed. By compressing the music, the cut grooves were not as wide, and with special cutting equipment, they could put more grooves on the disk. Yes... yes... I'm sure someone will point out the in fact there is only one groove on the disk. But the space of adjacent grooves could be closer with more compressed music.

But, no, as has been pointed out from multiple perspectives, the fact that the needles travels less distance on the inner groove is irrelevant to any thought of compression or loss of data in the music. The cutting needle, which is traveling the same speed and distance as the playback needle, is more than able to cut the data accurately into the disk.

Let me ask you this though :)rolleyes:), if a 30hz note is 11.44 meters long (37.5 feet), how can it possible fit on to such a small record?

In fact, how can such a long sound wave fit into my very small ear? Shouldn't my ear be at least 40 feet long to hear the frequency range it does?

Just kidding.

Steve/bluewizard
 
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Andy8421

Active Member
Steve,

The linear speed of the disc past the stylus has everything to do with the amount of data that can be cut into it. In a similar way to tape speed, the maximum frequency that can be recorded in a mechanical system is a function of the granularity of the recording material and the speed it passes the detector.

However, as has been pointed out already, a vinyl LP at 33 1/3 rpm has the ability to record frequencies far above human hearing (quadrophonic discs used a 30Khz carrier I recall). So although the maximum frequency recordable at the centre of the disc is effectively half that of the frequency recordable at the edge, it is still well above a 20KHz hearing threshold.

Where vinyl does suck is in low frequency amplitude compression. Unless compressed, large amplitude low frequency signals caused such large excursions that the stylus would jump out of the groove. To avoid this, LF is attenuated prior to recording (using the RIAA curve), and then boosted at playback. This has the unintended consequence of boosting any rumble present in the transport mechanism at playback.

LF signals are also mixed to mono to give the stylus an easier time of it, so a stereo LP is effectively mono at low frequencies.

I believe that this LF limitation issue is one of the reasons that older recordings destined for vinyl were mixed to be relatively bass-light in comparison to modern recordings intended for digital transportation where there are no such limitations.
 
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Alan Mac

Well-known Member
As a result of the decrease in the velocity of the stylus relative to the groove, the frequency response magnitude at high audio frequencies decreases as the replay stylus moves from the outer to the inner grooves.

In an attempt to compensate for this, the high frequency response of the amplifier system driving the cutter head is usually gradually increased as the cutter progresses from outside to inside grooves.

However, this results in further increased noise and distortion when playing back the inner grooves (over and above tracking distortion etc.)


In the bad old days before the advent of the Compact Disc, when I listened to Vinyl LPs I generally found the music from the inner groove sections to sound noticeably more distorted than than that from the outer grooves.


Alan
 

Alan Mac

Well-known Member
As a result of the decrease in the velocity of the stylus relative to the groove, the frequency response magnitude at high audio frequencies decreases as the replay stylus moves from the outer to the inner grooves.

This Hi Fi World review frequency response graph illustrates the difference in frequency response between the outer grooves (red trace) and the inner grooves (white trace) of a vinyl LP record.

See Ortofon 2M red frequency response.

Ortofon 2M Red/Ortofon 2M Blu/Nagoka MP11 - Measured Performance

The magnitude response for the inner grooves starts to roll off above 5 kHz and is about 5 dB down at 20 kHz compared with the outer groove response.


Alan
 

karkus30

Banned
I thought there was a natural tendency towards reduced high frequency at the centre of the records, something to do with the way the cutting head works. Can't remember where I read that.

Not sure boosting it would be a good idea. Having the Devil tell me to do bad things with a lisp just wouldn't work for me.
 

formbypc

Active Member
In the bad old days before the advent of the Compact Disc, when I listened to Vinyl LPs I generally found the music from the inner groove sections to sound noticeably more distorted than than that from the outer grooves.

But the rest of it sounded SO good it made up for it, didn't it....?
 

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