5 ms·

Now there is something Bartosz Ciechanowski should write a blog post about! I also think something important is missing from this explanation: the vertical/hor

by lqet 4y ago

Now there is something Bartosz Ciechanowski should write a blog post about! I also think something important is missing from this explanation: the vertical/horizontal solution would not have been backwards compatible in two ways: (1) A mono needle would not have been able detect the vertical movement, resulting in a missing channel. (2) A stereo needle able to play such a record would not have been able to play a classic mono record, because the mono signal would have only been translated into a single channel (the left or right channel), and the other channel would have remained silent. (1) would've made stereo records unattractive for customers who already owned an existing mono player, and (2) would've made stereo players unattractive to customers who already had a substantial mono record collection. With the 45 degree solution, existing mono players were able to play stereo records (the horizontal movement is exactly the sum of the two channels), and stereo players were able to play existing mono records. For this to work, the left and right signals were recorded in opposite phase. A really elegant solution, which somehow reminds me of the cover of GEB [0]. PS: if you are interested in cutting-edge record groove technology, the "Füllschriftverfahren" invented by Eduard Rhein might interest you. It was an early compression technology for audio. The method was based on earlier work by the London-based Columbia Graphophone Company, but their work was never used in practice. Basically, before this invention, the spacing between the grooves on a record was fixed, with enough margins so that large amplitudes would not cut into neighboring grooves. Rhein build a machine that dynamically spaced the grooves based on the maximum local amplitude, allowing much smaller groove margins for quiet parts of an audio file, and therefore increased information density. This nearly doubled the running time of typical records. Sadly, I only found an extensive description of this technology in German, including original patents [1]. But the figures are self-explaining. [0] https://i.stack.imgur.com/OKBvZ.jpg https://i.stack.imgur.com/OKBvZ.jpg [1] https://grammophon-platten.de/page.php?530 https://grammophon-platten.de/page.php?530

PopAlongKid 4y ago
>For this to work, the left and right signals were recorded in opposite phase. I don't know, but found the following comment in an audiophile/collector forum, note the comment about phase. https://www.discogs.com/forum/thread/774378?message_id=7697060#7697060 https://www.discogs.com/forum/thread/774378?message_id=76970... "When Stereo records first appeared circa 1958, it was important that they were NOT played on existing mono equipment, as the two playback modes were incompatible with regards to wear and tear. A mono player would damage the stereo record primarily because a stereo record required the stylus to wiggle up and down as well as left and right. Mono only required left and right, and mono players of the time had little up and down capacity. "Mono record players (or more specifically cartridges and stylii) then incorporated the ability to play stereo records and modern, of the time (late 1960's) players could therefore play a stereo record without damage. At the same time mono records were phased out, so people needed to be assured they could still purchase records (which were only available in stereo) and play them without causing damage. "It has nothing to do with the record, everything to do with the playback equipment. "As a further point stereo records were limited by phase - they needed to ensure that at various critical frequencies there was no difference in phase between left and right (bass is critical), otherwise the stylus might jump out of a groove. Some digital formats have recordings that contain much out of phase information - creating a holographic type impression. If these contain low frequency sounds out of phase, then these need to be modified prior to be prepared for release on a record. "
- starkparker 4y ago
  This is why there are still mastering engineers who specialize in vinyl. See also https://badracket.com/vinyl-mastering/ https://badracket.com/vinyl-mastering/ for more vinyl-specific mastering concerns.
- jlarcombe 4y ago
  Yeah the phase thing is quite a pain, I always thought it was just for really low stuff, but a band I was helping out had a master rejected by the pressing plant for excessive out-of-phase audio even when the passages in question weren't particularly 'heavy'. Since then I've been quite paranoid about it before sending masters off, and it's surprising how many mixes have a load of out-of-phase stereo. It can usually be tamed with some M/S EQ but then you miss the spaciousness in headphones...
  - eternalban 4y ago
    What causes the phase shifts?
    - jlarcombe 4y ago
      Sorry for delay in replying. Two main causes in my experience: 1. as a deliberate effect, either on its own or as part of a reverb; similarly, some synth sounds have this 'built in' 2. as a consequence of multi-microphone recording, where two microphones are used on the same source but are positioned such that the channels end up significantly out of phase The trouble is that inter-channel phase is one way in which stereo 'works' so it's all a bit of a compromise... this is why there are various tried and trusted spaced stereo microphone techniques that give a sense of spaciousness without introducing too much out of phase content (e.g. ORTF) and often these give a more pleasing result that the coincident-pair (XY) techniques that minimise phase differences. Often in situations where there's lots of sources being recorded at once you can end up with problems. The usual rule of thumb is to ensure that if two microphones are picking up a single source, the more distant one is at least three times the distance of the closer one to the source. A quick way to check this stuff with headphones when positioning microphones is to sum them to mono, reverse the polarity of one of the mics and move them around to minimise the bass cancellation.
jameshart 4y ago
> A really elegant solution, which somehow reminds me of the cover of GEB The thing is, when you play a record on a GEB phonograph, you run the risk of your phonograph vibrating itself apart.
timmg 4y ago
> (1) A mono needle would not have been able detect the vertical movement, resulting in a missing channel. (2) A stereo needle able to play such a record would not have been able to play a classic mono record, because the mono signal would have only been translated into a single channel I think the way they do it with FM radio is: one channel is A+B and the other is A-B. The A+B channel is the "mono" channel for backward compatibility. (Then they add/subtract to get left/right: (A+B)-(A-B)=B; (A-B)+(A+B)=A.) Pretty elegant solution, IMHO.
- phkahler 4y ago
  I don't recall why or how, but the A-B channel is of lower bandwidth, or otherwise more likely to drop out. Hence when you're starting to lose FM, you may find it dropping to mono.
  - timmg 4y ago
    I think the reason is less interesting: when you start to go out of range, your radio thinks there is only a mono signal, so it switches to mono mode. Same reason "static" on your TV is black and white. Your TV didn't detect a color signal, so it is looking for the black and white one. (For anyone old enough to remember static on their tv :)
- trelane 4y ago
  Indeed: https://en.wikipedia.org/wiki/FM_broadcasting https://en.wikipedia.org/wiki/FM_broadcasting "For this reason, the left (L) and right (R) channels are algebraically encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels through the single loudspeaker. A stereo receiver will add the difference signal to the sum signal to recover the left channel, and subtract the difference signal from the sum to recover the right channel." Other systems were considered by the FCC, but this one won (see article for the details) If you're interested in this sort of thing, the book Defining Vision is a great discussion of the history of HDTV.
justsomehnguy 4y ago
> Rhein build a machine that dynamically spaced the grooves based on the maximum local amplitude, allowing much smaller groove margins for quiet parts of an audio file, and therefore increased information density. This nearly doubled the running time of typical records. Just like SMR in HDDs
Lio 4y ago
The phase shift is really useful if you’re a DJ playing out on a loud system with a good chance of mechanical feedback. i.e. there’s a chance your cartridge will pick up sound from the speakers. The feedback will be picked up exactly the same on both the L and R channels so if you combine them to make mono the inverted channel will cancel out the feedback on the non-inverted almost perfectly. The stereo image isn’t usually useful in a venue setting anyway so there’s not much downside.

by lqet 4y ago

PopAlongKid 4y ago

>For this to work, the left and right signals were recorded in opposite phase. I don't know, but found the following comment in an audiophile/collector forum, note the comment about phase. https://www.discogs.com/forum/thread/774378?message_id=7697060#7697060 https://www.discogs.com/forum/thread/774378?message_id=76970... "When Stereo records first appeared circa 1958, it was important that they were NOT played on existing mono equipment, as the two playback modes were incompatible with regards to wear and tear. A mono player would damage the stereo record primarily because a stereo record required the stylus to wiggle up and down as well as left and right. Mono only required left and right, and mono players of the time had little up and down capacity. "Mono record players (or more specifically cartridges and stylii) then incorporated the ability to play stereo records and modern, of the time (late 1960's) players could therefore play a stereo record without damage. At the same time mono records were phased out, so people needed to be assured they could still purchase records (which were only available in stereo) and play them without causing damage. "It has nothing to do with the record, everything to do with the playback equipment. "As a further point stereo records were limited by phase - they needed to ensure that at various critical frequencies there was no difference in phase between left and right (bass is critical), otherwise the stylus might jump out of a groove. Some digital formats have recordings that contain much out of phase information - creating a holographic type impression. If these contain low frequency sounds out of phase, then these need to be modified prior to be prepared for release on a record. "

starkparker 4y ago
This is why there are still mastering engineers who specialize in vinyl. See also https://badracket.com/vinyl-mastering/ https://badracket.com/vinyl-mastering/ for more vinyl-specific mastering concerns.
jlarcombe 4y ago
Yeah the phase thing is quite a pain, I always thought it was just for really low stuff, but a band I was helping out had a master rejected by the pressing plant for excessive out-of-phase audio even when the passages in question weren't particularly 'heavy'. Since then I've been quite paranoid about it before sending masters off, and it's surprising how many mixes have a load of out-of-phase stereo. It can usually be tamed with some M/S EQ but then you miss the spaciousness in headphones...
- eternalban 4y ago
  What causes the phase shifts?
  - jlarcombe 4y ago
    Sorry for delay in replying. Two main causes in my experience: 1. as a deliberate effect, either on its own or as part of a reverb; similarly, some synth sounds have this 'built in' 2. as a consequence of multi-microphone recording, where two microphones are used on the same source but are positioned such that the channels end up significantly out of phase The trouble is that inter-channel phase is one way in which stereo 'works' so it's all a bit of a compromise... this is why there are various tried and trusted spaced stereo microphone techniques that give a sense of spaciousness without introducing too much out of phase content (e.g. ORTF) and often these give a more pleasing result that the coincident-pair (XY) techniques that minimise phase differences. Often in situations where there's lots of sources being recorded at once you can end up with problems. The usual rule of thumb is to ensure that if two microphones are picking up a single source, the more distant one is at least three times the distance of the closer one to the source. A quick way to check this stuff with headphones when positioning microphones is to sum them to mono, reverse the polarity of one of the mics and move them around to minimise the bass cancellation.

jameshart 4y ago

> A really elegant solution, which somehow reminds me of the cover of GEB The thing is, when you play a record on a GEB phonograph, you run the risk of your phonograph vibrating itself apart.

timmg 4y ago

> (1) A mono needle would not have been able detect the vertical movement, resulting in a missing channel. (2) A stereo needle able to play such a record would not have been able to play a classic mono record, because the mono signal would have only been translated into a single channel I think the way they do it with FM radio is: one channel is A+B and the other is A-B. The A+B channel is the "mono" channel for backward compatibility. (Then they add/subtract to get left/right: (A+B)-(A-B)=B; (A-B)+(A+B)=A.) Pretty elegant solution, IMHO.

phkahler 4y ago
I don't recall why or how, but the A-B channel is of lower bandwidth, or otherwise more likely to drop out. Hence when you're starting to lose FM, you may find it dropping to mono.
- timmg 4y ago
  I think the reason is less interesting: when you start to go out of range, your radio thinks there is only a mono signal, so it switches to mono mode. Same reason "static" on your TV is black and white. Your TV didn't detect a color signal, so it is looking for the black and white one. (For anyone old enough to remember static on their tv :)
trelane 4y ago
Indeed: https://en.wikipedia.org/wiki/FM_broadcasting https://en.wikipedia.org/wiki/FM_broadcasting "For this reason, the left (L) and right (R) channels are algebraically encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels through the single loudspeaker. A stereo receiver will add the difference signal to the sum signal to recover the left channel, and subtract the difference signal from the sum to recover the right channel." Other systems were considered by the FCC, but this one won (see article for the details) If you're interested in this sort of thing, the book Defining Vision is a great discussion of the history of HDTV.

justsomehnguy 4y ago

> Rhein build a machine that dynamically spaced the grooves based on the maximum local amplitude, allowing much smaller groove margins for quiet parts of an audio file, and therefore increased information density. This nearly doubled the running time of typical records. Just like SMR in HDDs

Lio 4y ago

The phase shift is really useful if you’re a DJ playing out on a loud system with a good chance of mechanical feedback. i.e. there’s a chance your cartridge will pick up sound from the speakers. The feedback will be picked up exactly the same on both the L and R channels so if you combine them to make mono the inverted channel will cancel out the feedback on the non-inverted almost perfectly. The stereo image isn’t usually useful in a venue setting anyway so there’s not much downside.