TDM and RTAS Plug-In Delay Compensation

[ LiB ]

TDM and RTAS Plug-In Delay Compensation

The DSP algorithms used in both TDM and RTAS plug-in architectures require a certain amount of time to process. TDM plug-ins have such a low amount of processing time that it is not noticeable in most situations. Except in extreme cases, RTAS plug-ins, by the nature of their design, have any processing delays compensated for by the DAE engine in Pro Tools. The delay caused by DSP processing of any kind is called latency and will affect playback of audio.

In its subtler form, plug-in latency will affect tracks that are related acoustically, such as the different mics used to record one orchestra. If two mics are used to record an orchestra and a TDM plug-in is applied to one of those mics during mixdown , the audio channel processed by the plug-in will be delayed by a small amount, thereby affecting the time relationship between those two microphones. This time relationship, in a recording of this nature, is critical. Even a few samples of latency can cause a dramatic shift in the stereo image generated by those two microphones. This latency should be compensated for in some manner. The simplest way is to apply the same plug-in to the unprocessed channel. This plug-in can have all its parameters set in such fashion that it will not affect that channel in any way besides generating the equivalent latency of the other channel, maintaining a proper phase relationship between the two.

In extreme cases, plug-in latencies of several thousand samples can grossly affect the timing of such things as dialog and foley tracks. The most common occurrence of this is when using noise reduction plug-ins that require very long DSP cycles to accomplish. A good example is the Waves plug-in, X-Noise.

X-Noise generates 5120 samples of latency when used as an RTAS plug-in. This latency is enough to throw off lip-sync. As this plug-in is used to remove environmental noise from production audio, dialog will be affected the most. The latency in this situation does not affect the relationship between two audio tracks but rather the relationship between the audio and video. Since there is no way to apply an equivalent plug-in to the video track, another method must be used to compensate for this latency. The following steps outline a method that uses track slipping and alternate playlists to compensate for any amount of plug-in latency:

1. In order to compensate for plug-in latency, it is first necessary to determine the amount of latency that is present. Pro Tools offers a very simple way to find out what the cumulative delay is for all processing that occurs on one track. If several plug-ins are applied to one track, the latency generated by each plug-in will add to the total latency of that track. Pro Tools has a Display mode that shows the total latency in samples for all processing on each track. Command+click on the volume indicators in the Mix or Edit windows , as shown in Figures 8.37 and 8.38.

   Figure 8.37. The volume indicator in the Edit window. Comman+clicking here will cycle you from volume to peak level and finally to delay or latency value in samples.

   

   
   

   Figure 8.38. The volume indicator in the Mix window. Command+clicking here will cycle you from volume to peak level and finally to delay or latency value in samples.

   

   
   

2. Command+clicking on the volume indicator once will toggle the display showing the peak amplitude value for that track. Command+clicking on this again will cause the processing delay value to be seen. It is displayed in samples of delay as shown in Figure 8.39.

   Figure 8.39. The DSP delay display as seen in the Edit window. This can also been viewed in the Mix window.

   

   
   

3. Set the Nudge format to samples and manually set the value found in the delay latency display into the Nudge value as seen in Figure 8.40.

   Figure 8.40. Manually entering the latency value for the track into the Nudge setting.

   

   
   

4. Create a duplicate playlist of the track you are working with. Name this duplicate playlist with a suffix that describes the plug-in you are compensating for. For instance, if the track in question is named Dialog 1 and you are applying a Waves X-Noise plug-in on that track, name the duplicate playlist Dialog 1-XN to indicate that this playlist compensates for the latency of X-Noise. This will help identify which playlist has been altered for delay compensation and which is the original. If you remove the X-Noise plug-in, you should return to the original playlist to maintain correct timing of the track.

5. Select all regions on the duplicate playlist by clicking in it with the Selector tool and pressing Command+A (Select All). Be sure you have not inadvertently selected other regions on tracks that do not have the X-Noise plug-in. You only want to move the audio that will be processed by the plug-in you are compensating for.

6. Press the minus key once to nudge the selected audio earlier (to the right) in time by the nudge amount, which is equal to the delay value of the plug-in, in this case 5120 samples for X-Noise RTAS.

7. Press play and the audio should be back in sync, exactly where it was prior to applying the X-Noise plug-in.

This method of plug-in delay compensation yields accurate results with any amount of delay. The other method of applying equal amounts of delay to other tracks in the session has two disadvantages. One is the need for additional DSP to delay all the other tracks in a session to compensate for one track that is using a DSP intensive plug-in. The second involves the delay as it relates to video that is in sync with your session. In the case of X-Noise, the delay value of 5120 samples is close to three video frames of delay, which when applied to dialog will throw lip-sync out the window. If all tracks in the session are delayed by the three frames needed to compensate for X-Noise, then the whole sound track will now play back three frames late in relation to the video. This is obviously a problem. The track slipping method outlines above does not suffer from these dilemmas and is relatively simple to use.

[ LiB ]