Encoders & decoders
How an AVoIP system replaces a matrix switcher.
Chapter 7's matrix switcher is a single box with a fixed number of inputs and outputs. Once it's full, adding one more display means a bigger frame. AV-over-IP (AVoIP) replaces that box with a network: every source gets an encoder, every display gets a decoder, and an Ethernet switch sits in the middle.
How the pieces fit
- An encoder (transmitter) takes HDMI in from a source, compresses it if the platform needs to, and sends it onto the network as a stream of IP packets — usually multicast (Chapter 8).
- A decoder (receiver) joins the stream it's told to, turns it back into HDMI, and feeds its display. Many decoders can scale the picture to their display, like a matrix output scaler.
- Switching means telling a decoder to leave one stream and join another. A control system or the platform's management software sends that command; the network switch does the rest.
Because the “matrix” is now the network, the system grows one endpoint at a time, any source can reach any display on the network — across a floor or a campus — and the same infrastructure carries video walls, multiviewers, USB for keyboard and mouse, audio and control.
| Matrix switcher | AV-over-IP | |
|---|---|---|
| Size | Fixed frame; upgrade when full | Add encoders and decoders as needed |
| Distance | Extenders on each port | Anywhere the network reaches |
| Switching | Inside one box | Decoder joins a different multicast stream |
| Main risk | Running out of ports | Network design and configuration |
| Who's involved | AV team | AV and IT teams together |
- Size
- Matrix switcherFixed frame; upgrade when full
- AV-over-IPAdd encoders and decoders as needed
- Distance
- Matrix switcherExtenders on each port
- AV-over-IPAnywhere the network reaches
- Switching
- Matrix switcherInside one box
- AV-over-IPDecoder joins a different multicast stream
- Main risk
- Matrix switcherRunning out of ports
- AV-over-IPNetwork design and configuration
- Who's involved
- Matrix switcherAV team
- AV-over-IPAV and IT teams together
Platforms and standards
AVoIP is not one technology. Encoders and decoders from the same platform work together; different platforms usually don't. The main families you'll meet:
- Manufacturer platforms built for installed AV, often with very low latency and uncompressed or lightly compressed video on 1 or 10 Gb networks. Several are built on shared chipsets or alliances, such as SDVoE.
- IPMX, an open set of standards for pro AV built on the broadcast industry's SMPTE ST 2110, aiming for interoperability between brands.
- Software and production protocols such as NDI, widely used in streaming and live production, and Dante AV, which extends Dante audio networking to video.
Each makes different trade-offs between bandwidth, latency and picture quality — the subject of the next lesson.
Where the bandwidth goes
Each encoder's port carries one stream out; each decoder's port carries one stream in. The links that need planning are the uplinks between switches. With multicast, an uplink carries one copy of each source that anything on the far side is watching — not one per decoder. With unicast, it carries a copy for every decoder.
36%
Comfortable headroom.
4
One copy per source watched, however many decoders watch it.
900 Mbps
Fits a 1 GbE port.
Bitrates vary widely by platform and settings, from tens of megabits for heavily compressed video to around 10 Gbps uncompressed — use the manufacturer's figures. The 80% warning is a cautious rule of thumb, not a standard.
Designing an AVoIP system
- Choose the platform from the content: resolution, frame rate, latency tolerance and the need for USB or audio.
- List every encoder and decoder, with the stream bitrate from the manufacturer.
- Pick switches the manufacturer has validated, with enough port speed, multicast features and uplink capacity.
- Agree VLANs, IGMP snooping and querier, QoS and addressing with IT (Chapter 8).
- Plan how switching will be controlled: control system, management software or both.
Key takeaways
- 01AV-over-IP replaces a matrix with encoders, decoders and a network switch.
- 02Switching means a decoder leaving one multicast stream and joining another.
- 03Systems grow one endpoint at a time and can span any distance the network reaches.
- 04Endpoints from different platforms usually don't interoperate; IPMX aims to change that.
- 05With multicast, an uplink carries one copy per source watched — size it for the worst case.
Check your understanding
Q1In an AVoIP system, what actually happens when a user 'switches' a display to a new source?
Q2Ten decoders on a far switch all watch the same source, sent as multicast. How many copies cross the uplink?
Q3Why should uplink capacity be calculated for every decoder watching a different source?