Volume Triggered Recording Guide

What this is about

When you record a source you often are only interested in the sound and not the silence in between. The recording trigger of rockbox provides you with a tool to automatically distinguish between sound and silence and record the sound only.

Introduction

Unfortunately it is not very easy to make this distinction between silence and sound because you hardly ever encounter real silence. There always are background noises. What is considered as background noise depends on the situation. For example during a lecture the very low noise of rustling paper might be considered as background noise. During a rock concert the murmour of the audience might be concidered background noise which is much louder compared to rustling paper. Also the duration of the signal matters. When you record speech you want to record every syllable. When you record live music you may not be interested in that chord the guitar man stroke two minutes before the show to verify his amp is turned on. The trigger features numerous parameters to adapt its behaviour to the desired situation.

For more technical short description see also VolumeTriggeredRecording

The Parameters

Trigger

This parameter specifies the trigger mode and can have one out of three values.

off	The recording is started manually, apart from the Prerecord time no other parameter has any effect
once	The trigger triggers one recording only. After the recording has finished the input signal can not start another recording.
repeat	The trigger can trigger multiple recordings.

Prerecord time

This specifies the time that is included into the recording before the trigger event occurs. This is very useful if you record a signal that fades in. Usually you want to set the prerecord time >= start duration. That ensures that you record the entire sound. Strictly speaking the prerecord time is not a special parameter of the trigger. It is available during normal recordings too.

Start threshold

The start threshold defines the minimal volume a sound must have to start the recording. It is displayed numerically in the line "Start above". Note that the unit of the thresholds depend on the settings of the peak meter. If the peak meter displays db you can adjust the level in db too. When the peak meter is set to linear the threshold is displayed as percentage. In the peak meter at the bottom of the screen the start threshold is displayed graphically by a little triangle pointing to the right. There are two special values. The value "off" turns the start condition off. With this setting you have to start the recording manually and the trigger only stops the recording according to the stop condition. The setting "-inf" sets the trigger to the absolute minimum. This setting only makes sense when you record via the digital input as even the self noise of the device would exceed this threshold immediately.

Start duration

The start duration defines the minimal duration that a signal must exceed the start threshold to start the recording. Depending on your situation you may want to set this setting to 0 (e.g. when copying a song from a commercial medium) or to quite big values. Because sound is not continuous by nature (think of percussion) neglectable dropouts are tolerated during this start duration.

Stop threshold

When the sound level drops below the stop threshold the recording is stopped. It is displayed numerically in the line "Stop below". Just like the start threshold the unit of the stop threshold depends on the settings of the peak meter. There's also a small triangular marker in the peak meter at the bottom of the screen. In contrast to the start threshold marker it points to the left. The value "off" turns the stop condition off. With this setting you have to stop the recording manually.

Stop hold

This time specifies the duration the signal must drop below the stop threshold to stop the recording. By selecting high values you ensure that eg. trailing fade outs are recorded entirely.

Presplit gap

When the signal drops below the stop threshold for the time specified by the presplit gap a new recording may be started when the signal raises above the start threshold. Thus the value of the presplit gap should be smaller than the stop hold time. Otherwise the recording would stop anyway and the presplit gap has no effect. For most uses I recommend to set this parameter equal to the stop hold time. Sometimes you may encounter a sound source (eg. a cd) where the songs have fade outs and hardly any gaps between the tracks. Here you can set the stop hold time to long values to ensure that all fade outs are recorded completely. By specifying a short presplit gap you still can split the recording into seperate tracks whenever the trigger start condition is met.

The trigger status display

The status of the trigger is displayed in the trigger settings dialog and in the recording screen. It is symbolized by a stop icon, a record icon and a progress bar. The icons display the recording state. When the recorder is recording the record icon is displayed, otherwise the stop icon. The progress bar represents either the start duration or the stop hold time. When it fills from left to right it displays the start duration and the recording won't be triggered until it is filled completely. During the stop hold time it fills from right to left. When the progress bar is filled completely the recording stops. In the trigger settings dialog the trigger status display is just a simulation. No recording is triggered yet.

Ready

The recorder is not recording. It waits for the signal to exceed the start threshold.

Steady

The recorder doesn't record to a file yet. The signal has exceeded the start threshold but not yet long enough (start duration) to start the recording. The progress bar fills from left to right to symbolize the time that is left before the recording starts. If you have specified a prerecord time the signal may already be recorded to memory and be stored to the file when the recording finally starts.

Go

The recorder records to a file.

Post recording

The recorder still records but is about to stop the recording. The signal has dropped below the stop threshold. If the signal remains below the stop threshold until the progress bar has filled from right to left the recording stops. When the signal raises above the stop threshold again the recording continues.

Hints

Playback versus live signal

You can use playback of an existing file or a live signal to feed the peak meter in the trigger settings dialog. If you have similar recordings on your hard drive. To use a file just start the playback normally and enter the trigger settings dialog via the main menu -> Recording -> Recording Settings -> Trigger. To use a live signal enter the recording screen first and then use the menu or the F3 quick menu (recorder) to enter the trigger settings dialog.

Resetting the trigger status simulation

In the trigger settings dialog you can reset the trigger status simulation by hitting the F2 button (recorder only). This is useful if you have specified long post recording times and dont want to wait until the trigger simulation stops.