Converting vinyl to digital, watch the tone arm
After going through the VHS to digital process, which I lamented earlier I started wondering about the state of digitizing old vinyl albums and tapes is.
There are a few turntable/cd-writer combinations out there, but like most people today, I'm interested in the convenience of compressed digital audio which means I don't want to burn to CDs at all, and nor would I want to burn to 70 minute CDs I have to change all the time just so I can compress later. But all this means I am probably not looking for audiophile quality, or I wouldn't be making MP3s at all. (I might be making FLACs or sampling at a high rate, I suppose.)
What I would want is convenience and low price. Because if I have to spend $500 I probably would be better off buying my favourite 500 tracks at online music stores, which is much more convenient. (And of course, there is the argument over whether I should have to re-buy music I already own, but that's another story. Some in the RIAA don't even think I should be able to digitize my vinyl.)
For around $100 you can also get a "USB turntable." I don't have one yet, but the low end ones are very simple -- a basic turntable with a USB sound chip in it. They just have you record into Audacity. Nothing very fancy. But I feel this is missing something.
Just as the VHS/DVD combo is able to make use of information like knowing the tape speed and length, detecting index marks and blank tape, so should our album recorder. It should have a simple sensor on the tone arm to see as it moves over the album (for example a disk on the axis of the arm with rings of very fine lines and an optical sensor.) It should be able to tell us when the album starts, when it ends, and also detect those 2-second long periods between tracks when the tone arm is suddenly moving inward much faster than it normally is. Because that's a far better way to break the album into tracks than silence detection. (Of course, you can also use CDDB/Freedb to get track lengths, but they are never perfect so the use of this, net data and silence detection should get you perfect track splits.) It would also detect skips and repeats this way. You could also do this with a small LED and optical sensor on the cartridge, if that can be non-standard. The inter-track area is of course very visible to our eyes, it's how we are supposed to find tracks by eye. I could also imagine such a sensor having an inside edge on detecting scratches. (Having out of band information is very useful in elimination of scratches and pops. That's because a general filter for these will distort the audio, so you want to apply it only over sections that you are pretty sure need it.)
The other thing I want is something I had as a kid -- a record changer. On the record changer you could stack 6 albums, and it would play them in sequence. Most of these came with very cheap turntables and cheap electronics, so they were totally disdained by audiophiles, but there is no reason you could not combine them with a decent cartridge, arm and turntable. That's what I want. To put 6 albums on, come back in one hour, flip the stack of six over and put it on the changer, and get the other six sides. (Except on Monty Python's Matching Tie and Handkerchief. Boy were we shocked the first time we discovered the gimmick on that.)
Come back in one hour for six sides? Yes, because we'll be playing all these albums at 78 RPM, or frankly as fast as you can safely play them without the cartridge skipping off (something it should detect by the way) and the digitizer should be good enough to sample at 44K samples per second of regular speed audio. (Oh, even 96K samples, which sound cards can do, at 78RPM gives you 41,025 samples/second which good resamplers can convert to the CD rate.)
Perhaps I'm asking too much, but it would be cool if the player noticed the arm did fly off or skip, and go back and retry at a slower speed. A basic retry of the whole album is easy. Being able to seek back in the album to retry requires a bit fancier mechanical control and would increase the cost. Indeed, since the changer gives me plenty of time (it may be simpler to view this as an overnight thing, so you have 8 hours to digitize 2 hours of music on 6 sides) it may make more sense to forget about the 78RPM mode and just do it super slow, which could produce better quality with less skipping. Both modes may make sense.
Now I say six sides because this was typical for the changers. In reality perhaps we can do much better, though after a while the weight gets high. But if the cheap-ass changers we had in the 60s could do it, I don't see a problem today. Of course if you could duplicate a jukebox mechanism and put large numbers of albums in, it would be great. In that case it might make more sense to make it expensive and rented rather than cheap and sold to everybody.
It should be possible just from the ratio of track lengths to find the closest matches to a set of track lengths in the CDDB database and provide an easy menu to identify albums. Or even a single pixel optical scanner in the tone arm could, if lifted, scan the label in the center of the album to provide an image and even do some pattern matching and OCR.
Other parts of the turntable can be very cheap. For example, we need not care a great deal about wow and flutter. If we have dots on the outside of the platter (similar to those used for strobe measurement) and an optical sensor, we can measure the exact speed of the platter very precisely at any given time. With that (and a high sample rate) we can resample dynamically (or adjust the sample clock in real time) to produce a result as though we had the most expensive, stable turntable in the world. We could get a result better than you could hear from the turntable in analog mode.
I know there's been research on optical cartridges that use a laser to read the physical bumps in the tracks, instead of a needle. There are even some very expensive laser turntables, though they are very sensitive to dirt in the grooves. That's great, and if it becomes cheap, I want it. But to make this box cheap, standard cartridges are the way. Let's face it, I'm not going to be playing any of these albums again, I would even tolerate it if I got a good recording and it made a little damage -- other than my desire for retries. But I would venture that most of the money would go into the cartridge and the tone arm. A2D at this sample rate are cheap, as are the optical sensors, and as were the record changer mechanicals.
(You can get record changers today for about $150, but they come with pretty low grade components. I would be curious as to how tolerable they are.)