Encoder settings for AWS Elemental

Which product do you have?

• A rack appliance running a web UI on your own network — You are using: Elemental Live · Path to Recast: RTMP, or SRT on 2.26.3+
• A channel configured in the AWS console — You are using: MediaLive · Path to Recast: RTMP output group
• A small hardware box feeding AWS — You are using: Elemental Link · Path to Recast: Feeds MediaLive; configure MediaLive

Elemental Link cannot send to Recast directly. It is a contribution encoder that feeds MediaLive, so configure the MediaLive channel it feeds.

Before you start

In Recast for Business, open Stream Inputs and choose + Stream Input. Pick RTMP or SRT — the choice is fixed once the input is created.

Recast then shows the values your encoder needs. Copy them from there rather than retyping them:

• RTMP STREAM URL — SRT STREAM URL
• STREAM KEY — STREAM ID
• SRT input shows: PASSPHRASE

Note that an SRT input labels its credential STREAM ID, not Stream Key. It is the same value, under the name SRT uses for it.

Then confirm all three of these, or your stream will be refused:

1. The stream input exists and you have its credentials
2. The stream input is attached to a video
3. That video is not a draft

A new stream input is switched off. Recast switches it on only when it is attached to a live, non-draft video.

Check your Elemental Live version first

This matters more here than with any other encoder.

• Below 2.22 — No SRT output at all — use RTMP
• 2.22 to 2.24 — SRT output exists, but no Stream ID field — cannot reach Mux
• 2.25 to 2.26.2 — Stream ID field exists, but see the warning below
• 2.26.3 and later — Recommended

Mux identifies your stream by the SRT Stream ID. Without that field there is no way to tell Mux which stream you are, so Elemental Live below 2.25 physically cannot deliver SRT to Recast — the connection will be refused every time regardless of your settings.

Do not use 2.25 through 2.26.2 for SRT. AWS fixed a defect in 2.26.3 where the web interface could double-enter the Reliable TS Stream ID field (GLV-3713). A doubled Stream ID sends Mux a doubled stream key, and the handshake fails with an error that points nowhere near the real cause. If you are on 2.25–2.26.2 and cannot upgrade, use RTMP.

Version is shown in the Elemental Live web interface under the support or about screen. If you are not sure, ask whoever maintains the appliance — do not guess, because the failure mode is silent and misleading.

Settings for Recast

• Great — Resolution: 1080p · FPS: 30 · Video bitrate: 5000 kbps
• Good — Resolution: 720p · FPS: 30 · Video bitrate: 3500 kbps
• Works — Resolution: 480p · FPS: 30 · Video bitrate: 1000 kbps

• Video codec — H.264 (AVC)
• Profile — Main
• Rate control — CBR
• Keyframe interval — 2 seconds
• Audio codec — AAC-LC
• Audio bitrate — 128 kbps
• Audio sample rate — 48 kHz
• Audio channels — Stereo

Use no more than 50% of your measured upload bandwidth. On an appliance in a facility this is rarely the constraint, but on a remote site it usually is.

In Elemental Live, the keyframe interval is set as GOP Size in the video stream settings. With GOP Size Units set to seconds, enter 2. If your workflow uses frames, enter 60 for 30 fps or 50 for 25 fps.

RTMP setup

Which URL? Recast displays rtmp://global-live.mux.com:5222/app as your RTMP STREAM URL, and that works. This guide uses rtmps:// on port 443 instead: it is the same Mux ingest wrapped in TLS, so your stream key is not sent in clear text, and port 443 passes through restrictive venue networks that block port 5222. Use whichever your encoder supports.

Elemental Live

1. Open the Elemental Live web interface and create or edit your event.
2. In Output Groups, add an Adobe RTMP output group.
3. Set the destination URL: rtmp://global-live.mux.com:5222/app - European facilities can use rtmp://eu-west.live.mux.com/app instead.

1. Set the Stream Name to your Recast Stream Key, exactly as shown in Recast. Copy and paste it — do not retype it.
2. Add an output to the group and set the video and audio settings from the table above.
3. Save, then start the event.

Elemental Live supports both RTMP and RTMPS output. Use RTMPS if you want the contribution feed encrypted in transit — the settings are otherwise identical, and the only change is the scheme and port in the URL we give you. If your Elemental Live system is on an older software version and RTMPS is not available in the output group, plain RTMP is fully supported and nothing else in this guide changes.

MediaLive

MediaLive reaches Recast through an RTMP output group.

1. In the AWS console, open your MediaLive channel and edit it.
2. Under Output groups, choose Add and select RTMP.
3. Add an output, and set the destination:
   • URL — rtmps://global-live.mux.com:443/app
   • Stream name — Your Recast Stream Key
   MediaLive supports RTMPS outputs, so use rtmps:// here.

1. Set Connection retry interval and Num retries to give the channel room to recover — Recast holds your slot for 30 minutes.
2. Configure the video and audio encode settings from the table above.
3. Save and start the channel.

MediaLive channels normally have two pipelines for redundancy. Recast accepts one stream per stream input, so either configure a single-pipeline channel, or point only one pipeline at Recast.

SRT setup

Elemental Live

Requires version 2.26.3 or later. See the version table above.

SRT output lives in the Reliable TS output group, not the RTMP one.

1. In your event, go to Output Groups and add a Reliable TS output group.
2. Choose Add Output.
3. Set Delivery Protocol to SRT.
4. Set SRT Connection Mode to Caller. Mux listens; you call it.
5. Fill in the primary destination:
   • Primary Destination — srt://global-live.mux.com:6001
   • Latency — 500
   • Encryption — AES-128
   • Key Value / Passphrase — Your Recast PASSPHRASE
   • Stream ID — Your Recast STREAM ID

1. Save, then start the event.

Do not put the stream key in the destination URL as a query parameter. Elemental Live does not parse query parameters out of that field — it needs the dedicated Stream ID field, which is exactly why the version matters.

Two field-level traps

Encryption must be AES-128. Mux requires a 128-bit key. Elemental's Encryption field offers several key lengths and 256-bit is a tempting default, but it will fail the handshake with an unhelpful error.

Passphrase length is capped at 72 characters. Elemental Live accepts passphrases between 10 and 72 characters. Recast's SRT passphrases fall inside this range, but if you ever see the field reject a pasted value, check for trailing whitespace before assuming the passphrase is wrong.

MediaLive

MediaLive can send SRT directly to Recast. You do not need MediaConnect in front of it. Create an SRT caller output group and point it at the endpoint on your Recast stream input.

Fill in the destination as follows:

• Connection mode — Caller. Recast listens; MediaLive makes the call.
• Destination URL — the SRT endpoint from your Recast stream input, including the port.
• Stream ID — the Stream ID from your Recast stream input. The field is optional in MediaLive, but Recast needs it, so fill it in.
• Encryption — MediaLive always encrypts SRT output, so this is not optional. Choose AES 128 and store the passphrase from your Recast stream input in AWS Secrets Manager, then select that secret in the destination. AES 192 and 256 will be refused by our ingest.
• Latency — set this to at least 500 ms, or four times your round-trip time to the endpoint, whichever is higher.

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Confirm you're live

1. Start the event or channel.
2. Check the Elemental Live event status or the MediaLive channel alerts — both surface connection errors quickly and specifically.
3. Look at the video in Recast. Allow 20 to 30 seconds: Recast uses standard latency, so the delay is expected and is not a fault.

Known quirks

Filler NAL units and CBR. Before Elemental Live 2.25.2, an RTMP output group stripped filler NAL units from CBR video, which can make a stream that is nominally constant bitrate behave as variable downstream. From 2.25.2 the RTMP output group has an Include Filler NAL Units field. If you are on 2.25.2 or later and want genuine CBR into Recast, enable it.

Reliable TS is MPEG-TS. The SRT path wraps your output in an MPEG transport stream. That is what Mux expects, so no action is needed — but it is why the setting lives under Reliable TS rather than with the RTMP outputs.

The SRT option is not for MediaConnect. AWS notes that the SRT delivery option is intended for downstream systems other than MediaConnect. Recast is such a system, so this is the correct choice.

One protocol per stream input. A Recast stream input is created as either RTMP or SRT and cannot be switched. If you want to move Elemental Live from RTMP to SRT, create a new stream input of the right type.

A standard channel needs two destinations. MediaLive requires one destination for a single-pipeline channel and two for a standard channel. Recast issues one SRT endpoint per stream input, so a single-pipeline channel is the straightforward setup. If you need to run a standard channel, contact us before your event and we will issue a second stream input, or use RTMP instead.

The passphrase has to be 10–79 characters. MediaLive enforces this. Recast passphrases sit inside that range, but if you regenerate one and it falls outside, regenerate again rather than trimming it.

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Troubleshooting

Event runs, but Recast shows nothing

Check the stream input is attached to a video and that the video is not a draft. This is the most common cause and it is invisible from the AWS side — Elemental will happily report a healthy output.

Then wait a full 30 seconds before concluding anything.

MediaLive SRT output connects, then drops at the handshake

Likely cause: Encryption set to AES 192 or 256
What to do: Change the output group to AES 128 and restart the channel

Connection refused or SRT handshake failure

1. Version below 2.25 — no Stream ID field, so SRT cannot work. Use RTMP.
2. Version 2.25–2.26.2 — the Stream ID may be doubled by the UI bug. Upgrade to 2.26.3, or use RTMP.
3. Encryption not AES-128 — Mux requires a 128-bit key.
4. Stream ID in the URL instead of the Stream ID field — move it.
5. Protocol mismatch — an SRT Recast input needs SRT; an RTMP input needs RTMP.
6. Stream key wrong — re-copy from Recast, and check for trailing spaces.

Output starts, then drops repeatedly

On an appliance this is usually the network path rather than bandwidth. Check whether UDP 6001 outbound is permitted if you are using SRT; many facility firewalls allow TCP freely and restrict UDP. Switching to RTMP over TCP is a fast way to test that theory.

You have 30 minutes to reconnect before the stream is treated as ended.

Viewers report stuttering

Check the event's dropped-frame and output-buffer statistics in the Elemental Live interface, or the MediaLive OutputDroppedFrames metric in CloudWatch. If frames are being dropped, reduce bitrate or resolution. If they are not, look at the source — a 25 fps source into a 30 fps encode causes visible judder.

Audio missing or out of sync

Confirm the audio encode is AAC-LC at 48 kHz stereo. Elemental Live will happily encode formats Mux does not accept, and the symptom is silence rather than an error.

For drift, prefer audio embedded in the SDI signal over a separate analogue or AES input, so audio and video share a clock.

Getting help

Include your exact Elemental Live or MediaLive version, the output group type, the protocol, and the verbatim error. For SRT problems the version is the first thing Recast support will ask for.

More detail: Recast live ingest reference

Sources

• AWS Elemental Live: Delivering TS using the SRT protocol
• AWS Elemental Live: Creating the SRT output
• AWS Elemental Live: Working with SRT
• AWS Elemental consolidated release notes — Stream ID field in SRT outputs (2.25, SOCK-38864); StreamID double-entry fix (2.26.3, GLV-3713); RTMP filler NAL units (2.25.2)
• MediaLive: Creating an RTMP output group
• Mux: Use SRT to live stream

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