In the world of broadcast, the term 'Constant Bitrate' (CBR) is common. However, it's essential to understand its application and its different contexts - video encoding and transport streams, as their definitions and technical implications vary significantly.
Understanding Constant Bitrate (CBR)
At its core, a Constant Bitrate refers to a method of data production that produces the same amount of data per second for the entire duration of the broadcast. This consistency provides predictability which is useful for specific applications, particularly in network planning, clock stability and decoder compatibility. However, the use of the term CBR can mean different things when referring to video encoding versus transport streams.
CBR in Video Encoding
In video encoding, CBR is a method where each video frame is encoded with approximately the same amount of data. This ensures that the data flow is mostly continuous and consistent. Some encoders have a CBR setting that is actually a very tight VBR band but does not actually guarantee that the bitrate will be constant with time. This can cause issues when the downstream equipment is expecting a CBR signal.
CBR in MPEG Transport Streams (MPEG-TS)
Unlike video encoding, when CBR is referred to in the context of MPEG-TS, it doesn't refer to the video encoding data rate being constant, but rather to the transport stream's overall data rate. The transport stream can carry packets of many different types of data, including video, audio and captions. The bitrate of each data type can vary with time.
A CBR MPEG-TS must ensure the total bitrate remains constant throughout the stream. To achieve this, MPEG-TS encoders and multiplexers transmit can "NULL" or "Stuffing" packets, when there is nothing to send, ensuring that the total number of bits per second remains constant.
As a result, even if the stream content bitrate fluctuates, the overall transport stream bitrate remains constant by inserting the requisite number of NULL packets.
Benefits of CBR Transport Stream for Broadcast Contribution and Distribution
Using a CBR transport stream offers several advantages, especially in the context of broadcast contribution and distribution:
1. Decoder Compatibility: CBR transport streams ensure compatibility across a range of decoders, including older models, reducing playback issues for media partners - particularly those with legacy playout infrastructure.
2. Clock Stability: CBR Transport Streams offer precisely timed data flow, ensuring clock stability, which is essential for synchronized commentary and smooth playback.
3. Network Compatibility: CBR transport streams match the data flow expected by many network infrastructures, reducing the chances that your media partners (takers) are unhappy with the signal they receive.
Spalk Requirements and Troubleshooting
1. You are not sending CBR to Spalk.
If you are sending SRT to Spalk, it must be a CBR MPEG-TS. The Spalk Dashboard will display this error message if you are not sending CBR SRT. You will have significant issues during the transmission if it is not CBR.
2. There is a very low NULL packet bitrate in the transport stream.
Your event is using SRT with CBR, but the CBR transport stream being sent does not contain enough NULL packets per second. You will likely have significant audio degradation during the transmission. To fix this, increase the NULL bitrate (sometimes called stuffing) in your encoder's MPEG-TS settings.
Some common software encoders have recently started supporting CBR, for more information try their helpdesk articles:
If you are trying to understand if your encoder supports constant bitrate please feel free to contact your Spalk representative and we can help test the output.
In summary, the primary benefit of using a CBR transport stream for broadcast contribution and distribution is the predictability it provides. This predictability can help ensure a smooth, high-quality broadcast, better network resource management and reduces incompatibility errors downstream.
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