Recording a Mix on a VTR

[ LiB ]

Even though many video layback operations are performed in a video post-production facility, it is preferable to lay back the audio yourself if you have access to a compatible video tape deck. Sony Digital Betacam is the current industry standard, though analog Betacam SP is still widely used. Laying back audio to videotape decks requires SMPTE synchronization and compatible audio connections.

Synchronization can be as simple as feeding analog SMPTE into your SMPTE to MTC converter and allowing Pro Tools to lock to that incoming time code. The Machine Control option can provide another level of functionality by allowing you to remotely control the videotape deck. Either way, Pro Tools and the videotape deck must be in perfect sync when laying back final mixes . This requires hardware that will resolve the speed of Pro Tools to the VTR. Refer to Chapters 3 and 4 for more detailed information on synchronization.

Using Digital Connections

When laying back to Digital Betacam, it is preferable to use digital I/O instead of going through an analog conversion stage. As Digital Betacam is capable of recording four tracks of 20 bit, 48kHz audio, several choices are possible for stereo or surround sound recording.

When printing a stereo track, it is possible to separate the dialog stem on Track 3 and the music and effects stem on Track 4, both in mono. This gives options to editors who will use that videotape later. You could also print an encoded multi-channel mix such as Dolby E on two tracks and a stereo mix (Dolby Pro Logic ) on the remaining two tracks. Be sure to properly label your tape so that anyone can see exactly what's on each track.

NOTE

DOLBY E

There is also a data compressed format known as Dolby E that encodes a 5.1 mix into a stereo pair of AES digital signals. This format is used to carry multichannel audio over existing two-channel cabling and equipment and survive multiple encode and decode stages before broadcasting. Like Dolby Digital, which is primarily used for broadcast signals, Dolby E requires a dedicated encoder and cannot be listened to directly without decoding. With a Digital Betacam VTR, Dolby E can be encoded onto Tracks 1 and 2 while a stereo downmix is recorded on 3 and 4 as a reference you can listen to without decoding. The TV series 24 uses this method of layback, providing broadcasters with six-channel discrete mix on a four-channel VTR including a stereo compatible reference mix. Isn't technology great?

If you are mixing in 24-bit resolution, it will be necessary to dither your outputs to 20 bits to avoid truncation errors and quantization noise. Since this is a digital format, you will not need to worry about distortion as long as your levels do not reach or exceed 0dBFS. Keep in mind, however, that a digital master tape might be used to create analog copies, whereas peak levels beyond -8dBFS might cause distortion. Make sure you know exactly what the master tape will be used for.

Professional digital videotape decks will have AES digital inputs and outputs. This type of digital connection is the most reliable. Figure 9.3 shows the rear of a Digital Betacam deck wired up for digital I/O. The AES ins and outs have transformers in them that convert the XLR connection to a coaxial BNC type that can run longer lengths from the machine room to the audio suite in this facility.

Figure 9.3. The rear connections of a Digital Betacam deck shown with BNC transformers connected to the AES ins and outs in the lower right hand corner.

Your Pro Tools system must be resolved in some way to the video tape deck or to the same master sync reference to which the video deck is resolved. To resolve Pro Tools to the video deck using a Sync I/O or Universal Slave Driver, take a coaxial BNC cable and connect the analog video output from this deck to the video reference input of the Sync I/O, and then choose Video In as your clock reference in Pro Tools. To resolve to a dedicated "house sync" used by the video deck, connect the sync source to the Sync I/O and choose Video Reference as your sync source.

Another method utilizes the clock signal embedded in the digital outputs of Channels 3 and 4 to clock your Pro Tools system. Connect this AES signal to a compatible input of your Pro Tools interface and choose that digital input as your clock reference. If you are recording to Channels 1 and 2, their outputs cannot be used as a clock reference because that would create a digital loop. When recording to Tracks 3 and 4 of the VTR, it will be necessary to re-route the clocking channels to outputs of 1 and 2 for the same reason.

Using Analog Connections

When using analog connections to lay back to a videotape deck, make sure you aligned your reference tone of 1kHz@-20 dBFS to 0VU on the videotape deck. Also be aware that in analog recording mediums such as Betacam SP, peak levels that exceed -8dBFS begin to induce distortion artifacts in the recorded signal.

It will not be necessary to dither your outputs to lower bit resolutions than 24 in this case. Take advantage of the full dynamic range of your system when using analog connections. This might not make a noticeable difference, as the analog recording medium has a limited dynamic range, but it doesn't hurt.

With analog VTRs, it is still necessary to clock Pro Tools and the VTR together. As no digital clock source is available from the VTR, video or house sync will be used as a clock source.

NOTE

CHECK IT AGAIN

Always listen back to your master recordings from the VTR or other recoding medium. You must be absolutely sure that your mix recorded perfectly to the master. This is your last chance. Check it twice!

[ LiB ]