EEVblog #386 – Glass Delay Lines Part 2

Part 2 on Piezo-electric acoustic glass delay lines.
Some measurements and playing around.
Part 1 is HERE

Delay line info is HERE
(chapter 6.9)

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    • vampirefrog

      Hi Dave! I really like your videos, they got me interested in electronics again, and I’ve started learning stuff.

      I can’t create an account on the forum cause I’m not receiving the activation email, and I’ve tried resending it too, so I’ll just ask here – how would you design a function/signal generator? Perhaps something like the μSupply or the μCurrent, a μFunction, that has some basic wave forms, and maybe even programmable.

    • Ian Broadwell

      I love your blogs and tech reviews they are awesome. I am after a VNA and wonder what is the possibility for reviewing a classic HP 8510c

    • Chris

      Hi Dave,

      i’m not surprised that it peaks around 13 MHz. Pixel clock frequency for PAL is about 13.5 MHz.



    • Christian Berger

      Actually that delay line is probably there for dropout compensation. The chroma path of Video 8 doesn’t need a delay line AFAIK.

      This is probably used for dropout compensation. So what you run through it is the FM-modulated video signal comming from the head. Once the amplitude drops below a certain value, it will switch into a loop mode and replace the video with the previous line. Just like your VHS recorder would do.
      That FM-signal is modulated like this: (for PAL)
      100% white 5.4 MHz
      sync tip 1.2 MHz
      But there’s a pre-emphasis with a tau of 1.3µs going up to 9.5 dB. There’s some clipping at 220% white and 90% black.
      The modulated signal occupies the space between about 2 and 9 MHz.

      Video 8 has little use for a 4.43 MHz delay line. BTW Hi8 goes up above 12 MHz for the modulated FM signal.

      BTW, if you want to see all the small reflections. You can lay a sync line between the generator and the scope and set the scope to average. You can average out the noise that way.

      • Christian Berger

        Ohh I may need to correct myself. Video 8 VCRs with external CVBS inputs might use such a delay line to separate chroma from luma, but that seems unlikely in a consumer camcorder.

    • Nick

      Dave, mike also did a test of a delay line,

      and I definetly think that the “reflection” as you could see and told us was some wired stuff, is just the reflection from the previous period, you had the period time 100 us, or whatever it was, but you could see it clearly with a linger period time.

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