1. Comparison of SRT and NDI Low Latency Transmission Protocols:
About SRT:
SRT is an internet transport protocol co-created by Haivision and Wowza, and it is currently a very popular open-source low-latency video transmission protocol. By using SRT transmission technology, it successfully enables secure and reliable high-definition video transmission and distribution over ordinary internet environments across multiple locations. It is now widely used in embedded live streaming devices, especially in broadcasting-related products!
About NDI:
NDI is a network device interface protocol launched by NewTek in 2015, which is a signal transmission protocol based on local area networks. Using NDI transmission technology, a device within a local area network can output or receive multiple NDI signals through a single network cable, completely replacing traditional SDI/HDMI video cables. It has made efficient video transmission in the IP space a reality.
SRT and NDI Bandwidth Usage:
The SRT protocol is a transmission protocol based on H.264/H.265 compression schemes, with adjustable transmission bandwidth. For example, high-definition video can be custom-set between 4-20Mbps. SRT's anti-packet loss mechanism is the ARQ (Automatic Repeat reQuest) error correction mechanism. In network transmission, bandwidth jitter and packet loss typically occur randomly, and error correction mechanisms are only needed when network issues arise. The ARQ mechanism simply requires the sender to retransmit lost data packets after packet loss occurs, ensuring transmission quality while reducing unnecessary bandwidth consumption.
There are two versions of NDI: NDI|HX and Full NDI. NDI|HX is a variant of H.264, currently iterated to NDI|HX2, with adjustable transmission bandwidth, where 1080P60 video defaults to 10Mbps. Full NDI uses I-frame intra compression schemes, currently iterated to NDI 4.5. During transmission, Full NDI requires a certain bandwidth, with high-definition bandwidth ranging from 90-180Mbps and 4K bandwidth from 200-300Mbps. It is recommended to use a gigabit network for NDI transmission, suitable for applications with high image quality requirements, such as program production work scenarios.
SRT and NDI Transmission Latency:
SRT is a network transmission technology capable of real-time and accurate data stream transmission in complex network environments. It uses the UDP protocol at the transport layer, featuring the fast speed and low overhead of UDP transmission, supporting point-to-point transmission without the need for intermediate server relay (only one end needs to have a fixed public IP address). Internet point-to-point transmission can be less than 1 second. With a packet loss rate below 20%, SRT can control latency between 200ms and 500ms, which is significantly lower than the 2 to 5 seconds latency of the RTMP protocol.
NDI allows devices within a local area network to transmit signals with high quality. A single device on the local network can output or receive multiple NDI signals through one cable, eliminating the need for complex wiring. The end-to-end latency within the local network is only 1-3 frames (less than 100ms). In practical applications, Full NDI has an end-to-end latency of less than 100ms, while NDI|HX has an end-to-end latency of approximately 100+ms.
SRT and NDI Application Scenarios:
SRT can be widely used in remote production of programs (cloud-based), video connections for live events at sub-venues, internet-based remote teaching and training, video supervision of construction sites by corporate groups, remote connections for court hearings, and other scenarios requiring remote video transmission over the internet. It is important to note that SRT transmission applications require either the sending or receiving end to have a fixed public IP address.
NDI is widely used in local/remote production of television programs, NDI screen mirroring, NDI video conferencing, ultra-low latency surgical demonstrations, and other scenarios that require convenient, low-latency, high-quality video transmission. Currently, NDI is only suitable for transmission within a local area network, but through protocol conversion technology, NDI streams can also be transmitted across the internet!
Comparison between SRT and RTMP Internet Transmission Protocols:
RTMP Protocol
RTMP stands for Real Time Messaging Protocol. It is an application layer protocol proposed by Adobe, relying on a reliable transport layer protocol (usually TCP) to ensure the reliability of information transmission. RTMP is used for real-time transmission of video, audio, and data between servers and Flash players.
RTMP is currently one of the most widely used streaming media transmission protocols, extensively utilized in the live broadcasting field. It can be said that the vast majority of live streaming platforms on the market support this protocol.
Advantages of RTMP:
RTMP is a protocol specifically developed for streaming media, with optimizations at the lower layer that are superior to other protocols;
Wide support: Major domestic CDN platforms and OTT platforms generally support RTMP, and most hardware or software encoders also support the RTMP streaming format;
RTMP is suitable for long-duration playback, remaining relatively stable for continuous playback of over 10 days.
Disadvantages of RTMP:
The RTMP protocol requires an RTMP server for end-to-end video transmission, resulting in significant transmission delays. If a user builds their own proprietary RTMP server, the delay can be controlled to around 1 second. However, when using commercial RTMP servers for video transmission, the delay typically ranges from several seconds to over ten seconds.
RTMP has cumulative latency, which is due to the fact that RTMP is based on TCP and does not drop packets. Therefore, when the network condition is poor, the server will cache the packets, resulting in accumulated latency, which typically ranges from a few seconds to several tens of seconds.
RTMP is based on TCP transmission and uses the non-public port 1935. RTMP has historically been difficult to pass through firewalls;
RTMP is a proprietary protocol from Adobe, and many devices are unable to play it, especially on iOS, where a third-party decoder is required for playback;
SRT Protocol:
The SRT protocol (Secure Reliable Transport) is a new open-source standard protocol invented by Haivision. In 2017, Haivision, in collaboration with Wowza, established the SRT Alliance to promote the development and application of SRT.
The SRT protocol is a network transmission technology that enables real-time, secure, and reliable data stream transmission in complex network environments. It utilizes the UDP protocol at the transport layer. Although UDP is an unreliable transport protocol, SRT's powerful data recovery mechanism, combined with the inherent speed and low overhead of UDP, ultimately achieves secure, stable, and fast transmission results with SRT.
SRT is currently a very popular internet video transmission protocol, widely used in remote broadcasting for television programs, corporate live streaming, and internet streaming applications.
Advantages of the SRT Protocol:
Reliability: Adaptable to any network environment, efficiently handling network packet loss, jitter, and bandwidth fluctuations.
Low latency: Due to the use of UDP transmission and the ARQ packet loss recovery mechanism, the transmission latency level based on the public network can generally be controlled within 1 second.
High Quality: SRT's transmission and error correction mechanisms maximize the utilization of available bandwidth and eliminate network errors and interference. This allows for the transmission of higher bitrate video streams under the same network conditions. When combined with efficient encoding formats such as H.264 and HEVC, it ensures high video quality even in poor network conditions.
High bandwidth utilization: Unlike the multi-rate adaptive distribution technology of ABR, which requires additional bandwidth for redundant bitrate, SRT monitors network link status in real-time and can adjust the bitrate dynamically (NAE, Network Adaptive Encoding). Additionally, the ARQ packet loss recovery mechanism significantly saves bandwidth and reduces network congestion compared to TCP's packet loss recovery mechanism.
Safety: The SRT uses AES-128 or 256 encryption to protect content security;
Free and Open Source: SRT is completely free and open source.
Drawbacks of SRT:
SRT is based on a bidirectional UDP point-to-point connection, suitable for high-quality, low-latency reliable transmission between two points, but not very suitable for content distribution to a large number of users.