Fix: Implement Bidirectional Volume Sync and Prevent Feedback Loop

[GiacoBot]

This pull request addresses a feature gap where set_volume() in A2DP Source mode only updated the local volume state without synchronizing it to the connected sink device via AVRC. It also corrects a potential infinite feedback loop that could occur from the volume synchronization mechanism.

Problem

When the library is used in A2DP Source mode, there were two primary issues with volume control:

1. One-Way Synchronization: Volume changes initiated on the ESP32 using the set_volume() method were not transmitted back to the sink device. This resulted in a desynchronization where the ESP32's internal volume state updated, but the actual audio output level on the sink device remained at its previous setting.

2. Infinite Feedback Loop: The mechanism for receiving volume updates from the sink involves an AVRC notification (ESP_AVRC_RN_VOLUME_CHANGE) that, in turn, calls the set_volume() function. If set_volume() also sends an update to the sink, it creates a circular dependency:

   • set_volume() -> sends command to sink.
   • Sink changes volume -> sends notification to ESP32.
   • ESP32 receives notification -> calls set_volume() again.
     This would result in an infinite loop of commands and notifications, wasting resources and potentially causing instability.

Solution

To resolve this and provide a truly bidirectional and stable synchronized volume control, this PR implements the following changes:

1. Feedback Loop Prevention: A guard condition has been added at the beginning of the set_volume function:

   if (is_volume_used && volume_value == volume) {
     return;
   }

   It breaks the infinite loop by immediately exiting the function if the requested volume is the same as the current volume. This ensures that when the ESP32 receives a volume-changed notification from the sink for a volume it just set, it doesn't trigger another redundant command.

2. Integration of AVRC Command: Inside the set_volume method, a call to the ESP-IDF function esp_avrc_ct_send_set_absolute_volume_cmd() has been added. This function is responsible for sending the new volume level to the connected sink device via AVRC.

3. Conditional Logic for Safety: The AVRC command is wrapped in a conditional check:

   #if ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(4, 0, 0)
   if (isSource() && is_connected()){
       esp_avrc_ct_send_set_absolute_volume_cmd(0, volume_value);
   }
   #endif

   This ensures that the esp_avrc_ct_send_set_absolute_volume_cmd() is only executed when the instance is operating as an A2DP Source and is connected to a remote device

[pschatzmann]

I assume that you have tested this solution