High-frequency in-band call signals are often associated with party-line (PL) user intercom devices. Typically 20 kHz and contained within an audio channel, these signals can be used to activate (“trigger”) a visual and/or audible indicator on a user beltpack or console device. A number of Dante®-enabled devices from Studio Technologies support this type of call signal. At the present time, high-frequency call signal capability is typically not compatible with the auto mix function as implemented in the Studio Technologies’ Models 5421 and 5422A Dante Intercom Audio Engine devices. The reason is that the device’s auto mix action will reduce the signal level on channels that are not currently designated as an active or holdover “talker” channel. This auto null action works well for voice-band signals, keeping unwanted audio content from being sent to the user devices on the virtual PL bus. The downside is that a high-frequency call signal that originates on a user device will also be attenuated in level. In many cases, the resultant call signal level sent to the PL bus will be less than the “trigger” level required by a receiving device. So while a call signal may be present on a party-line channel, its level may be 10 to 20 dB less than is necessary to activate (“trigger”) a visual and/or audible indicator. Only a device whose audio channel is designated by the auto mix function as an active or holdover “talker” channel will present a call signal at full level.
Fortunately, this situation is far from dire. Should support for call tone activity be required for an application that uses a Model 5421 or a Model 5422A the solution is simple. On the Model 5421, use a configuration choice in the STcontroller application to disable auto mix operation. On the Model 5422A select the Party-Line group function and not the Party-Line w/Auto Mix function. Both of these selections will cause virtual Party-Line channels to be created without invoking the auto mix action. While the benefit of auto mix won’t be available in these situations, call signal operation will be correctly supported.
It’s expected that future versions of Model 5421 and Model 5422A main firmware will allow support for call signals when the auto mix function is active. Should this become a reality, this technical note will be revised accordingly.