Windows provides VCR services through a device driver that is based on the MCI
command set for VCRs. This section describes the MCI Video System Control
Architecture (VISCA) driver and explains how to use it to control a VCR.
device type controls VCRs. For a list of the MCI commands recognized by VCR
devices, see VCR Command Set
The MCI VISCA Driver
The MCI VISCA driver controls Sony VISCA-compatible VCRs, such as the CVD-1000
VDeck. The VISCA driver controls the tape transport, channel tuners, and VCR
input and output channels.
Searching and Positioning with a VCR
The VISCA driver uses two methods to track videotape movement within the VCR
tape transport: timecode information
and tape counters
. Timecode information is timing information that has been recorded on the
videotape. Most VCRs allow timecodes to be recorded without destroying audio and
video tracks. Tape counters estimate the amount of videotape that travels past
the videotape head to obtain a position.
Both timecode information and tape counters increase as the videotape moves
from beginning to end. Because of its accuracy, using timecode information to
position a videotape is almost always preferable to using tape counters.
The MCI command flags for specifying positioning information are expressed as
time dependencies: "time format", "duration", "from", "to", and "seek". (Also,
"position" command returns its time value in the current time format.)
The VISCA driver uses the set
"time mode" command to select the type of positioning to use with a videotape.
When the time mode is set to "timecode", the status
"position" and set
"time format" commands use the timecode on the videotape. When the time mode
is set to "counter", the status
"position" and set
"time format" commands use counters.
An application can set the time mode to "detect" if it doesn't matter that
there might be two sources of position information. When in detect mode, the VISCA
driver uses timecode information for positioning when any of the following
- The timecode information is present when the driver is opened.
- You change a videotape with the set "door open" command and timecode information is present on the videotape.
- The set "time mode" command is reissued.
If timecode information cannot be found, the driver uses the tape counters.
To determine the current positioning method, issue the status
"time type" command, which returns either "timecode" or "counter". You can
also identify the current positioning mode by using the status
"time mode" command, which returns "timecode", "counter", or "detect".
"counter" command retrieves the current tape counter value, regardless of the
current positioning method; however, you can use this counter reading only with
The VISCA driver can retrieve the native timecode format recorded on a
videotape by using the status
"timecode type" and status
"frame rate" commands together. For example, if timecode type is "smpte" and
frame rate is 25, the native timecode format recorded on the videotape is SMPTE
The VISCA driver can also retrieve the counter resolution by using the status
"counter resolution" command, which returns "seconds" or "frames". The counter
format might still be set to SMPTE 30, but the return value returns only a
frame of 0. If the current time type is counter, then this resolution applies also
to the value returned by status
Frame-capturing commands provide still images for a frame-capture device
. A frame-capture device is a separate piece of hardware capable of reading
and storing the video image. The VISCA driver supports the freeze
) command to stabilize a still image for capturing. Also, the unfreeze
) command can be used to restart the tape transport following a freeze
command provides a high-quality, stabilized, time-base
corrected image for a frame-capture device. This
command exists because a device might not always deliver its maximum-quality
output image during playback or while paused; such a video image is not suitable
command unlocks the tape transport and resumes the transport mode in effect
before the freeze
When your application needs to record a video image on the VCR, use the freeze
"input" command or the cue
) command to record the image.
The VISCA driver supports three input types: video, audio, and timecode. The
video inputs include two standard channels (lines 1 and 2), an SVideo channel,
an auxiliary channel, and a channel from an internal tuner. The audio inputs
include two standard channels (lines 1 and 2) and a channel from an internal
tuner. The timecode input is internal to the VCR.
The normal outputs carry the currently selected inputs when the VCR is
recording or when the tape transport is stopped, and they carry the contents of the
videotape when the tape transport is playing or paused. The monitored outputs
carry the same information as the normal outputs plus current timecode and channel
Assuming the appropriate external inputs are connected to your VCR and you
have decided what you want to record, you can select the inputs to be recorded.
For example, to record or view from the "svideo" video and the "line 1" audio
inputs, you would use the setvideo
) and setaudio
) commands to select these input sources. You can verify these selections by
using the status
By default, the monitor shows exactly what appears as the output. Sometimes,
however, you might want to view one source while recording from another. This is
a common practice using the tuner. For example, you might want to watch
channel 4 while you record channel 7. In this case, you have two logical tuner
inputs. You could set up the VCR by using the following commands:
To review one source while recording from another
- Use the settuner (MCI_SETTUNER) command to select the channels to watch and record.
- Use the setvideo command to select the video-recording source.
- Use the setaudio command to select the audio-recording source.
- Use the setvideo command to route the channel 4 video input to the monitored output to display
- Use the setaudio command to route the channel 4 audio input to the monitored output to play the
- Verify your selections by using the status command.
The VISCA driver also supports a special input type for audio and video called mute
. Mute allows the selection of "no input," which is useful when recording a
Selecting Recording Tracks
Three types of recording tracks exist on a videotape: video, audio, and
timecode. You have only one video or timecode track, but you can use more than one
audio track. When you do so, make track 1 the main audio track.
The VISCA driver supports two operating modes: assemble and insert. In assemble mode
, all tracks are selected to be recorded. In insert mode
, tracks can be independently selected for recording. Most VCRs are in
assemble mode by default. Use the set
) command to change these modes.
Recording and Editing
) command provides simple recording and is accurate to approximately 1 second
of the starting position. To record more accurately, or if you expect to edit
the video content while simultaneously operating multiple decks, you should use
command prepares the device for recording or playing. Use the cue
"input" command to prepare the device for recording. The cue
command is required because an application must know when the device is ready
to perform the command (and because it can take several minutes to prepare for
) or record
The VCR prepares itself for recording or playing by seeking to the in-point
, which is the current position or the position specified by using the cue
command. If the "preroll" flag is specified with the cue
command, however, the VCR positions itself the preroll distance from the
in-point. The "preroll" flag also indicates that the VCR uses any applicable
editing mode, so it's important that you use "preroll", especially when you want the
most accurate recording. (Use the capability
) command with the "can preroll" flag to check whether the preroll mode is
When you record using "from" and "to" positions, the "from" position is
included in the edit, and the "to" position is not.
For more information about recording, see Recording
Using the Clock While Editing
When editing, you might want to record segments from one VCR to another. You
can begin recording at a specific time and position on one VCR while another
begins playing at the same time and position by specifying an action (play or
record), a position, and a time for each VCR.
Both VCRs must use the same clock for this type of editing; the clock helps
synchronize both devices. You can determine if two VCRs share the same clock by
using the status
) command with the "clock id" flag to query each VCR. If the identification
numbers returned by the status
command are the same, the devices use the same clock. As a shared resource,
the clock can be connected to multiple VCRs. The VISCA driver supports only one
You can also determine clock resolution by using the status
"clock increment rate" command. This command returns the number of increments
the clock supports per second. For example, if the clock is updated every
millisecond, the command returns 1000 as the clock increment rate. The advantage of
using the increment rate is that the rate is expressed as an integer;
otherwise, the increment would be a (single- or double-precision) floating-point value.
As an integer, manipulating the increment rate is a simple operation and is
not susceptible to rounding errors. You can reset the clock by using the set
) command with the "clock 0" (zero) flag.
When issuing a play
), or seek
) command, you can specify when the command is to be executed. The
characteristics of the VCRs being used determine when to start each VCR. The timing must
account for the amount of preroll each device requires and the amount of time
needed to complete the MCI commands used to set up the edit session. To do this,
retrieve the clock time and add a waiting interval of 5 to 10 seconds. (The
waiting interval must be long enough to let the preroll and any outstanding MCI
commands finish executing.)
To ensure that the waiting period is long enough, place the record
command last in your application and check the time immediately before it. If
the interval is too short, restart the play
command. Alternatively, you could check the time immediately after the last
command of the script to verify that there is enough time to send and complete
all the commands.
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