I took pictures of timecode...

[ccsnd]

For those who have never seen it, I took some screen shots of timecode. This is 23.976 TC. each bit of timecode is either 14 or 27 samples long at 48k. (there are long bits and short bits.)

[Jay Rose]

Exactly what LTC should look like. Biphase signal with 2400 Hz clock (at 30 fps). If there's no reversal within the 2400 wave, it's a 0. If there's a reversal (turning it into two 4800 Hz waves), it's a 1.

So it's really not a question of long bits and short bits. A long complete wave is "zero" bit. TWO short complete waves is a "one" bit.

Generated as a squarewave by the sending device, but designed to survive all sorts of degradation through an analog audio channel. That's why it's only the zero-crosses that count.

80 bit word. 26 bits are the time location as BCD, 16 bits for serial word sync and tape direction detect, balance for format identifiers and user bits.

Pretty darned clever, considering when it was adopted.

[chrisnewton]

Great. thanks CCalandro.

[ccsnd]

Yeah, I was over simplifying it way too much...

Here is from Wikipedia.

http://en.wikipedia.org/wiki/Linear_timecode#Longitudinal_timecode_data_format

[Matt Mayer]

Exactly what LTC should look like. Biphase signal with 2400 Hz clock (at 30 fps). If there's no reversal within the 2400 wave, it's a 0. If there's a reversal (turning it into two 4800 Hz waves), it's a 1.

So it's really not a question of long bits and short bits. A long complete wave is "zero" bit. TWO short complete waves is a "one" bit.

Generated as a squarewave by the sending device, but designed to survive all sorts of degradation through an analog audio channel. That's why it's only the zero-crosses that count.

80 bit word. 26 bits are the time location as BCD, 16 bits for serial word sync and tape direction detect, balance for format identifiers and user bits.

Pretty darned clever, considering when it was adopted.

Oh, does this bring back bad memories.

For a time during 744 development, I was decoding recorded timecode waveforms by hand to check for accuracy. Not something I want to do on a regular basis anymore...

[Philip Perkins]

It brings back VERY bad memories of examining analog-recorded TC from early attempts at CTTC 1/4" machines with Dan Dugan using Sound Tools (predecessor to ProTools). We were about discovering that some brands of 1/4" tape in use in those days had so many micro-dropouts that the TC signal (@ 7.5 ips) would not read properly. I'm not a techie, but I ended up with an oscilloscope for the express purpose of looking at TC to see if it was locked and or had a goofed waveshape. Let's hear it for digital TC!

philp

[Philip Perkins]

Chris if you have some pix of bad TC that would be great to see--ie jittery, unlocked, bad unreadable wfms etc

philp

[Jay Rose]

Jittery or dropouts might be noticeable in a waveform. But unlocked, bad formats, etc could still look like the good stuff as a combination of 2400 Hz and 4800 Hz waves. You'd have to actually count the bits, and compare them to a map of the proper bit pattern.

With modern equipment, level shifts or extreme band limiting might look ugly on a scope but be perfectly readable.

Philip, back in the Old Days what kind of synchronizer or reader were you using? IIRC some of the 1980s devices tried to square up the wave and look at it as a logic signal (either + or zero) rather than counting crossings. So distortion of the wave could be a problem.

CTTC was an FM signal with a high frequency carrier, wasn't it? So another layer of potential problems with the tape surface (and even head clogs), compared to LTC on a baseband audio track, and what you were actually looking at in SoundTools was the output of the playback demodulator...

Sprocket holes were so much easier.

[Philip Perkins]

Jittery or dropouts might be noticeable in a waveform. But unlocked, bad formats, etc could still look like the good stuff as a combination of 2400 Hz and 4800 Hz waves. You'd have to actually count the bits, and compare them to a map of the proper bit pattern.

With modern equipment, level shifts or extreme band limiting might look ugly on a scope but be perfectly readable.

Philip, back in the Old Days what kind of synchronizer or reader were you using? IIRC some of the 1980s devices tried to square up the wave and look at it as a logic signal (either + or zero) rather than counting crossings. So distortion of the wave could be a problem.

CTTC was an FM signal with a high frequency carrier, wasn't it? So another layer of potential problems with the tape surface (and even head clogs), compared to LTC on a baseband audio track, and what you were actually looking at in SoundTools was the output of the playback demodulator...

Sprocket holes were so much easier.

We could see the shift in the size of the areas of the different size wfms w/ badly unlocked code, and you would never be able to get it to settle down on a scope. Some ugly code was readable, some ugly code was re-shapeable some ugly code was unusable. The scope was a window, however low-rez, into at least some of what was happening with the code. The synchronizers were whatever the telecine depts had--anything from Otari to Nagra to Adams Smith to Shadow etc etc. That was part of the problem--many companies rolled their own, then had issues, so we needed some info on what we were recording in order to properly participate in the assignment of blame party. We were after something that would work as often as possible without me getting a phone call from telecine in the middle of the night. CTTC was just an analog audio track, with bias, like any other. We tried having no bias and we tried FM modulating the track so that it could coexist with the stereo Nagra FM pilotone track in case the TC track didn't work. (Not my idea--the combining of the two practically guaranteed that neither the TC or the Pilotone would work right.) What we saw in Sound Tools looked pretty much like what you see in Chris's post (but in black and white of course).

I didn't like analog TC much, especially after the simplicity of pilotone, but very few people in my area ever mastered mag-film technology at all, and even in my early days it was considered too expensive and labor intensive. I don't miss mag.

philp