[email protected] (Byron A Jeff) wrote in
--
-> In article <
[email protected]>,
->
-> ====
-> I'm trying to get wider coverage to my questions. The original
thread -> can be found in sci.electronics.basics - BAJ
-> ====
->
-> In article <
[email protected]>,
-> --> -
-> -> Gert van den Heuvel wrote:
-> ->> -> ->>
-> ->>
-> ->>>Basically I'm designing it using a 4029 to feed the control
inputs -> of ->>>the 3 4051s, to switch between up to 8 AV inputs. I
might later -> use ->>>some additional logic to allow the 4029 to
reset itself when -> it gets ->>>to a certain output, to limit the
switching between a -> certain number ->>>of inputs. The second output
I'm just planning to -> manually switch ->>>between the inputs with a
rotary switch, so it's -> possible a video ->>>signal will be switched
2 ways at the same time, -> but unlikely. So I ->>>need to put one of
these buffers on both of the -> outputs? ->>>
-> ->>>Nick
-> ->>>
-> ->>
-> ->>
-> ->> OK, so you have 8 inputs and 2 outputs. 1 output is switched
using -> a ->> 4051 and the other ouput is switched between the same 8
inputs -> with a ->> rotary switch.
-> ->>
-> ->> I assume all 8 inputs are grounded via 75 ohms resistors (to
-> minimize ->> reflection)? In that case you need buffers on both
-> outputs. Yep, even ->> on the output with the rotary switch. It you
-> don't, the internal ->> impedance of the device on the output will
-> become parallel-connected ->> to the 75 ohms resistor at the input.
-> ->>
-> ->>
-> ->
-> -> I haven't terminated the lines, although I've been reading and
it -> -> seems like a good idea. Does the 75 ohm resistor go from
input to -> -> ground/shield, or just in series with the input?
-> ->
-> ->
-> -
-> -The resistors go from input to ground.
->
-> This is the magic thread for me. This is the next hardware project
on -> my list. It must be the fact that the new television season is
right -> around the corner.
->
-> I'm planning on using 74HC4051s to switch video and stereo line
level -> audio. The thread has been helpful, but I have 3 issues:
->
-> 1) What is the setup for line level audio? Does it require the
-> impeadance
-> matching resistor too?
-> If so then what value?
-> Is an audio buffer required?
->
-> 2) I know it's silly, but I presume that all of the video/audio
-> shields are to
-> be grouded
-> to the 74HC4051's ground right? The reason I ask is while
-> researching this task I came across Zoomcat's parallel port
-> switcher here:
->
->
http://www.geocities.com/zoomkat/ppswitcher3.htm
->
-> On the page he has the following statement:
->
-> -----------
-> All the grounds (RCA plug outside) are connected together,
keeping -> audio and video seperated. Note that there are no
connections made -> to the ground side of the parallel port
connector (that would -> ground out the video and audio signals).
The chip grounds are -> connected to the common video out (not
video ground) going to the -> capture card.
-> -----------
->
-> And in fact on the diagram it does show the 74HCT259 ground
-> connected to the common video out.
->
-> 3) The primary reason for using the HC4051 instead of individual
-> transistors
-> was for packaging reasons. The video (and possibly audio)
buffers -> will cost another transistor per output (which certainly
isn't as -> bad as a transitor per input!). However it would
simplify my task -> if I could use an opamp package with a circuit
such as: ->
->
http://ourworld.compuserve.com/homepages/g_knott/elect254.htm
->
-> #2 the only confusing thing I've found about the whole process. Any
-> insight would be appreciated.
->
-> BAJ
->
->
-1) There's no audio buffer required. The input resistance of of audio
-devices is usually 10k or more. The 4051 just adds 500 ohms or so.
This -also means that there's no need to terminate the audio lines.
Cool. That simplifies the task.
-
-2) Yes, all audio and video grounds must be connected to pin 7 of the
4051. -The digital ground is connected to pin 8.
Now this raises another question. What voltage should Vee (pin 7) be
relative to the digital ground. Should it be -5V?
-Zoomcat uses an NPN transistor to switch audio and video. The ground
of the -74HCT259 is NOT connected to ground, but to the emitter of the
switching -transistor, which is connected to the center (signal) pin
of the RCA -connector.
I can see what he did. What I don't understand is why he did it. It's
important because from your statement below a trasistor based buffer
seems to be the ticket.
-
-3) You only need to buffer the video signal. Opamps for video
frequencies -are quite expensive, but if that's not an issue, feel
free to use them.
Since it's only the outputs for the video (which will end up being
only two or possibly 3) transistors will be fine. Also the audio
doesn't need it. So I think I'll go the transistor route.
So here's what I know:
1) Video needs to be buffered.
2) Video inputs needs to be terminated with 75 ohm resistors.
2) Shields should be connected to Vee.
Here's what I'm still confused about:
1) Should Vee also be tied to the power supply?
2) If so should it be -5V or 0V?
3) Will Zoomcat's transistor buffer be sufficient?
4) If so then what is the relationship of the digital and analog
grounds for
the transistor?
For the most part I'm a digital circuits dude. The concept of grounds
in pretty simple in the digital world. But I'm tres confused about
this mixed signal stuff. If I were using relays as an isolated
curcuit, then of course there is no issue. But the 4051 and the
transistors create a voltage relationship between the input, output,
and digital parts of the circuit, and frankly I get lost without the
concept of connected grounds.
Let's use Zoomcat's second switcher schematic here:
http://www.geocities.com/zoomkat/pix/para9.jpg
as a sample of what I mean. It's clear that he's using the transistor
as a switch. In a typical digital senario the base emitter current
relationship is established by grounding the emitter or tying the
collector to Vcc. In either case raising the base voltage turns on the
switch. But in Zoomcat's circuit there is doesn't seem to be a voltage
relationship between the three transistor components other than the
fact that the collector and emitter share a common ground. So how does
changing voltage on the base cause movement in the emitter?
Doing what Zoomcat says not to do "grounding the video" would make
sense to me. By tying the shared ground to the digital ground of the
parallel port, then a clear voltage relationship between the
transistor components can be established. When the voltage in the base
is raised relative to the now common ground, the emitter will follow
the collector relative to the now common ground.
But the way that he has it on the full schematic:
http://www.geocities.com/zoomkat/pix/swmulti3.jpg
I find completely baffling. Grounding the emitter establishes the
relationship between it and the base. I'm fine with that. But that
means that the center conductor of the output is fixed to ground
right? So then how in the heck does it work?
This is what happens when you become a computer
scientist/microcontroller engineer instead of listening to your
parents and becoming an electrical engineer!
BTW the OP of this thread stated that he tied all of the audio, video,
and Vee all to ground and it worked great. Is that in fact the ticket?
Any insight would be greatly appreciated.
BAJ