Transport Stream Mode For Outputs

The transport stream (TS) mode selects whether the output is constant bitrate (CBR) or variable bitrate (VBR). It is controlled using advanced settings described below.

An advanced settings property will be placed in the Properties panel when set to a non-default value, and placed in the Advanced Settings drop-down panel when set to its default value. The settings are described in the Stream Processing tab of the Multiplexer Parameters.

Transport stream mode for ASI outputs

The TS mode for ASI outputs is always CBR and cannot be changed.

Configuring the transport stream mode for IP outputs

The TS mode for IP outputs and the number of packets per frame are configurable from Beam version 1.12. Prior to that version, the TS mode for IP outputs is always CBR, and there are always 7 packets per frame.

The TS mode for IP outputs may be CBR or VBR. It is a single setting that applies to all IP outputs. The default is CBR. The supported VBR mode is SMPTE ST 2022-3, and hence the TS will only be valid if it contains no more than one referenced PCR PID.

When the TS mode for IP outputs is set to CBR, null and non-null packets populate each IP frame. IP frames will be output regularly at a frame rate determined by the total bitrate of the multiplexer and the number of packets per frame.

When the TS mode for IP outputs is set to VBR, non-null packets populate each IP frame. All null packets are discarded, providing maximum latency requirements are met (see Controlling latency for VBR TS IP outputs). IP frames will be output at a frame rate that is no greater than the equivalent CBR frame rate. The total bitrate of the multiplexer determines the maximum bitrate of the VBR TS payload of each IP output.

To configure CBR TS mode for all IP outputs:

1. Click on the transport stream in the Output Stream panel.

2. Find the TS mode for all IP outputs property, either in the Mux tab of the Properties panel or in the Advanced Settings drop-down panel.

3. Select CBR from the TS mode for all IP outputs property.

To configure VBR TS mode for all IP outputs:

1. Click on the transport stream in the Output Stream panel.

2. Find the TS mode for all IP outputs property, either in the Mux tab of the Properties panel or in the Advanced Settings drop-down panel.

3. Select VBR - SMPTE ST 2022-3 (Single referenced PCR PID only) from the TS mode for all IP outputs property.

Ensure the output TS contains no more than a single referenced PCR PID. If the output TS contains multiple referenced PCR PIDs, the output TS will not be compliant with ISO/IEC 13818-1.

Controlling latency for VBR TS IP outputs

The TS packet period for a CBR TS is constant. When encapsulated in IP frames, there is a latency of between 0 and 6 TS packets. This latency is usually small - for example, a 10 Mbps TS encapsulated in a 7-packet IP frame would incur a latency of less than 1 ms.

The TS packet period for a VBR TS is not constant. For a low VBR TS bitrate, the TS packet period will be high. After IP encapsulation, the IP latency could be much higher than that of the equivalent CBR TS. The maximum VBR frame period guards against excessive latencies.

The maximum VBR frame period is a single setting that applies to all IP outputs. The default is 100 ms. It defines the maximum time between consecutive VBR frames. Null packets are inserted in IP frames if necessary to meet the maximum VBR frame period.

This setting is necessary for the following reasons:

• Certain protocols such as RTP, FEC and RIST must buffer data, and equipment using those protocols expect a minimum frame rate to ensure a reasonable latency through their buffers.

• Downstream equipment is likely to declare a loss of input if a frame does not arrive within a certain period after the previous frame arrived.

• The latency through the buffers, both in downstream equipment and the multiplexer output buffer, can be controlled directly via the maximum VBR frame period.

Note that the latency for IP outputs is increased by the maximum VBR frame period. The default is sufficient for most applications, and it is only necessary to change the maximum VBR frame period to reduce the latency for IP outputs and/or reduce the maximum latency through downstream equipment input buffers.

For low-latency applications, there is a trade-off between latency, packets-per-frame and the minimum TS bitrate before nulls are allowed into a VBR IP output. For example:

• Using the default of 7 packets per frame, the default maximum VBR frame period of 100 ms results in a minimum TS bitrate of approximately 105 Kbps. The maximum latency through a downstream RTP buffer of 20 frames is 2 seconds.

• Reducing the maximum VBR frame period to 10 ms increases the minimum TS bitrate to approximately 1 Mbps. The maximum latency through the same downstream RTP buffer is 200 ms.

• With the maximum VBR frame period still at 10 ms, reducing frame occupancy to 1 packet per frame reduces the minimum TS bitrate to approximately 150 Kbps.

To configure the maximum VBR frame period for all IP outputs:

1. Click on the transport stream in the Output Stream panel.

2. Find the Maximum VBR frame period for all IP outputs property, either in the Mux tab of the Properties panel or in the Advanced Settings drop-down panel.

3. Set the Maximum VBR frame period for all IP outputs property to the desired value in milliseconds.

To configure the number of TS packets per IP frame:

1. Click on the transport stream in the Output Stream panel.

2. Find the Number of TS packets per IP frame property, either in the Mux tab of the Properties panel or in the Advanced Settings drop-down panel.

3. Set the Number of TS packets per IP frame property to the desired value.