All about signal flow


Video #1 - Graphic overview

Video#2 - Jumbotron Signal Flow

Video #3 - 8 Channel Mackie


Let's think of the human cardiovascular system.   See a graphic of it here.   

You know the basics: the heart pumps oxygen-rich blood out through arteries.  Then the oxygen-depleted blood returns to the heart through veins.   At the "heart" of the system, of course, is the heart.

  1. Think of the human cardiovascular system as a metaphor for audio/radio and video/television production systems.  

  2. The "heart" of an audio/radio studio is the "air board" or "audio mixer."

  3. The "heart" of a video/television production studio is the "production switcher," which is linked to the "audio mixer."  One manipulates video signals, and the other manipulates audio signals.



Okay, let's apply that "heart" idea to a simplified drawing of an audio system for radio.  See a graphic here.

  1. The "Audio Console" is the heart.  
  2. On the back of the console are three "inputs," one from a microphone, one from a CD player, and one from an audio recorder.  Notice that the signal can only go one way from a microphone and from a CD player, neither being able to record.  But the "audio recorder," which could be a tape machine or hard disk drive, is able to play (thus creating input to the console) or record (thereby receiving output from the console).   See #4, below.
  3. The audio console is able to regulate -- with special on/off switches -- which of the three inputs is allowed to pass through to the output.  And, the console is able to regulate the signal strength of each of those inputs, using special volume controls.
  4. This console shows only two outputs.  They're basically the same, but directed to different applications.  The first is "program out," and it represents the master output of the console.  Its signal goes first to some "signal processing equipment" such as a limiter/compressor, and then to the transmitter.  
  5. The second output is split between headphones for the operator (needed especially when a microphone is turned on) and a loudspeaker.  Both are used to monitor the program being created.
  6. Of course, an audio recorder could be put in the line after the signal processing equipment to record the master output, after all enhancements.   In fact, this diagram shows by dotted line a signal path from the final signal processing device to the input section of the audio recorder.


Now let's move to a more complete picture of the "On-air audio/radio system."    See a graphic of it here.

  1. At the center is the mixer -- or board, which physically is installed in the audio booth.  Across the top of the mixer are shown the "input signals."  These come from lots of different sources, such as microphones, CD players, networks, and so on.  A basic radio system will be set up to handle about a dozen inputs, with 25-30 inputs being more common.

  2. The mixer then controls which of the inputs is mixed to output.  It also controls the volume level -- or loudness -- of each.  And, it can do some other things, such as control the overall or master level of output.

  3. These mixed signals then are sent to outputs, which are shown on the right side of the mixer. 

  4. Along the bottom of the mixer are shown several special-purpose channels.  One is to trigger the on-air light on the studio door.  Anytime a microphone is mixed to output, the on-air light should automatically be turned on.  The second integrates the studio clock to the mixer.  Third is for studio monitors, which are powerful loudspeakers driven by a power amplifier.   And fourth is an output jack in the board for plugging in the operator's headsets.

  5. On the left side of the mixer is shown a sophisticated power supply.  They're used because household power (a wall outlet) is too unstable.

  6. Notice that some inputs come from equipment that also are set to receive output from the board.  That's because a CD deck, for example, can be used to record the commercial or program, or it can be used to play back the commercial or program.

  7. And, notice that each of the four microphones sends its signal first to a "voice processor," which allows you to fine-tune the sound of the microphone before sending it to the mixer.



And now let's ramp up to a video/television system.    See a graphic of it here.

  1. The video/television system comprises several subsystems: audio (which is like the audio/radio system above), video, headsets or intercom, teleprompter, lighting and others.

  2. On the left side of the graphic you'll see three microphones whose signals go to inputs on the audio mixer.   Below the microphones is a "floor monitor speaker," which is an audio monitor for use in the studio.  That speaker takes an output from the mixer.

  3. Below the mixer is an audio cassette deck, a DAT deck (digital audio tape), and a reel-to-reel recorder.  Beside them are a desktop computer which operates Software Audio Workshop's audio/nonlinear editing system.  

  4. Shown at the lower right corner of the mixer is the master output, which sends signal first INTO the two videotape recorders, and then to the master output for the whole system.  Videotape machines also have their audio outputs connected to the audio mixer as mixer inputs, and their video outputs connected to the switcher as switcher inputs.

  5. Shown at the top of the audio mixer is a video program monitor, used by the audio operator to see what's going on in the video channel.  Also shown are a pair of small audio speakers, which are the audio monitors for use in the audio booth.

  6. Let's return to the left side of the graphic where the microphones are.  Above them are a program monitor and a "floor director headset."  The program monitor is used in the studio so everyone can see what's going on.  Big studios (news) also hang monitors on the front of the cameras so talent can see what's on-air through peripheral vision.  The headset, as shown, is a wireless intercom, connected to a push-to-talk microphone in the control room.  Because the floor director has to move around quickly in the studio, his/her headset is often wireless -- which means the floor director wears a little transceiver on his/her belt.

  7. Now let's skip up to the very top, left corner.  You see a spotlight.  Assume that's one of a hundred or so which are hung in the studio.  As shown, the lighting instruments are wired to the Lighting Control Board (light mixer).  The Lighting Control Board allows individual lights to be turned on or off, and to be faded up from off, or faded down and out.  In fact, the light instruments are connected to a back-of-house "dimmer pack" which actually turns up and down the electricity sent to lights; the Control Board is simply a device that controls the dimmer pack.  The Lighting Director also has a program monitor and a special headset channel (to listen to the director or to talk to lighting assistants elsewhere in the studio).

  8. Now let's continue with the video subsystem,  starting with the cameras.   You'll see that the output of each is wired to an input on the video production switcher.  The video production switcher interface panel is to the video production switcher as the lighting control board is to the lighting dimmer pack.  In our control room, the switcher (interface panel) is mounted on a desktop console, while the switcher itself is mounted in an under-console equipment rack.

  9. Shown above the switcher are four more inputs.  The character generator is a special-purpose computer that creates text, usually used as "supers."  In this graphic, the brand name of the character generator is "Inscriber."   Next is the still store or still file device, which stores on hard disk individual frames of video which you can capture by a mouseclick.  Frames of captured video often are used as backgrounds for production graphics.  Next to the still store is the computer graphics unit, in this case a "PaintBox" brand.   A computer graphics subsystem is simply a broadcast version of Photoshop, Illustrator and Premier.  Shown next to the computer graphics editor is the Digital Video Effects box, or DVE.  This equipment allows creation of special transitions: slide, flip, mosaic, snowflake, and others.   In reality, these inputs to the switcher are not connected as the graphic shows.  Cameras, for example, are wired to the Camera Control Units which are mounted in a rack at the video engineer's position.  The CG and computer graphics units usually are located somewhere in the control room, while the Still Store and DVE units are in a backroom rack (they're operated remotely at the switcher).

  10. From the lower right corner of the switcher are shown two outputs.  One is program, or the actual picture that will be transmitted live like news or be recorded like commercials.  The program output goes to a program monitor, to a waveform monitor, and a vectorscope, and to inputs on the two videotape recorders,  before being sent to the transmitter room.  The other output is preview, the picture that will be transmitted next, which is sent only to a monitor.

  11. The waveform monitor shows everyone how a signal looks in terms of blackness and whiteness.  The vectorscope shows how that signal looks in terms of its three primary colors (red, green and blue).  The video engineer continuously refers to these monitors to make sure the pictures all are consistent in white value, black value, and color.

  12. Before leaving the control room, let's go back and pick up the remainder of the headset/intercom subsystem.    Shown behind the three cameras in the studio are three wireless headsets for the camera operator.   In the control room are intercom stations for the director, the technical director (the person who switches), and the producer.  Beside the SAW in the audio booth is a headset/intercom station for audio.   An intercom station is shown for the person running the videotape machines and servers (often in a separate room because of noisiness).  And the final intercom station is shown for the person who is operating the Master Control Room.

  13. To the lower left of the switcher is shown a "Monitor Wall."  Actually, this wall of monitors is built in front of the production people who sit in production control.   The bottom row of monitors show current video from Camera #1, Camera #2, and Camera #3, as well as a space for an extra camera (such as one for a weather remote).   Above the camera monitors are Character Generator, Still Store, a computer, and the DVE (again, these simply show current video from these devices).  Above those are Videotape machines 1 through 4.  Most stations also have video servers replacing one or more videotape machines.  And in the top row are feeds from Edit Suite #1 and #2, and Satellite Feeds #1 and #2.

  14. Professional studios never number/label their inputs as this graphic shows.  Notice that this control room has Cameras 1 and 2, Videotapes 1 and to, Video servers 1 and 2, Edit suites 1 and 2, and Satellites 1 and 2.  That's redundant and clumsy.  More important, it's too hard to speak, and too easy to confuse.  Professional studios, ours included, number cameras 1 through 4, then number tape machines 11 through 20, then number video servers 22 through 24, and so on.  Or, sources can be named, not numbered: Moe, Larry and Curley.

  15. Now we can move downstream from production control to the Master Control Room.  A "master" is used so that ongoing programming can be transmitted while production people are busy in the production control room making programs, commercials, promos, and other materials to be recorded for later on-air use. 

  16. Master can choose from many inputs (which are outputs for production control and audio!).  He/she can air material from videotape machines, from an automated videotape machine system, from servers, and/or from network (which usually comes in via satellite).  The output of master control goes to the transmitter (known as terrestrial), and/or to a satellite uplink, and/or to the local cable companies' headends.  Signals sent to satellites are usually compressed first.

  17. Finally, this Television System graphic, as drawn, shows a "field production" subsystem.  Look at the very bottom of the graphic, left side.  You'll see a TV camcorder (configured for use in the field) and two microphones.  The mikes go into a little mixer which sets levels correctly, and then inputs the combined audio signal to the camcorder.  A videotape goes into the camcorder, and "field audio and video" are recorded on it.

  18. Next, the tape is put into an editing room videotape machine.  Hooked to that machine is a computerized editing system.  The various shots and audio tracks are copied to the computer, rearranged suitably, and then recorded out to another "Edited Master" videotape.   That Edited Master videotape can be taken to one of the control room videotapes, or it can be taken to a master control machine, for playback in a production or for playback on the air.  

  19. In these days of "tapeless" recording, you'll find that many videotape machines have been replaced by a "media server."  So, if the field production subsystem happens to be in the newsroom or in the production department, then one camcorder can be replaced in this graphic by a dozen or so physical camcorders.  Each of these would record on "P2 cards," not on videotape.  P2 cards currently hold from 4 to 64 gigabytes, with 64 gb representing about 4 hours of recording time.  After recording, when the P2 cards are returned to the station, the raw video from the field can be "ingested" directly from the P2 card to a media server using a firewire connection.  The editing system can call up video and audio directly from the server for editing purposes.  The edited master then is simply rerecorded on the media server.  From the production or master control room, the server can be cued, and the story can be played directly to air.  That means the data file which the camcorder originally creates is allowed to move into the station, through editing and to an on-air status without ever having been recorded to tape.

  20. As shown in the graphic, there's an intercom/headset station in the news or production editing room, used so that the production or master control rooms can stay in touch with the video editor who is working on a soon-to-air piece.