VAST Required Media Files

Why VAST tags need multiple renditions

A VAST tag usually points to several media files at different bitrates so the video player can pick the right one for each viewer's connection and device. Mobile on a weak signal gets the low bitrate file; a smart TV on broadband gets the high bitrate file. When renditions are missing, players either stall, fall back to a lower quality version, or fail to render the ad entirely.

Adaptive playback only works if the VAST tag actually delivers the renditions your inventory expects. A tag that ships with one 3500 kbps mp4 and nothing else will starve mobile users. A tag missing the high quality rendition will look soft on a living-room screen. Either way, viewability, completion rate, and the advertiser's brand all take the hit.

What counts as a required media file

Most ad specs follow IAB's Digital Video Ad Format Guidelines and require three rendition tiers:

• Low quality: typically around 500 to 700 kbps, 360p source, for constrained networks
• Medium quality: typically around 701 to 1500 kbps, 480p source, the standard-definition fallback
• High quality: typically around 1501 to 3500 kbps, 720p or 1080p source, for HD and connected-TV environments

Some specs go further and require specific formats (video/mp4, video/webm), codecs (H.264 with a specific profile), resolutions, or VPAID and OMID API frameworks on top of the bitrate tier.

What this test checks

Advalidation downloads every media file declared in a VAST tag, including every VAST variation that rotates behind the same tag, and compares them against the rendition definitions in your ad specification. For each rendition you define, the test confirms:

• At least one media file matches the declared bitrate range
• Format, codec, resolution, and API framework match when specified
• The declared properties in the VAST XML match the actual file properties
• Duration is consistent across the renditions in a single tag

Because every variation is tested independently, a rotation of three creatives produces three per-rendition results. One missing high quality file in the third creative will not hide behind a passing first creative.

Configuring required renditions in your ad specification

In the ad specification editor, define each rendition as its own row under Media files. A description label, a format, an optional resolution, an optional API framework, and a bitrate range are enough for most specs. Three tiers labelled "Low resolution mp4", "Medium resolution mp4", and "High resolution mp4" with their corresponding bitrate ranges is the most common setup.

Then choose how strict the test should be:

• Require all of the media file definitions above: the VAST tag must contain a matching file for every rendition you defined. This is the right choice when adaptive playback depends on all tiers being present.
• Require at least one of the media file definitions above: any one of the renditions is enough to pass. Useful when a publisher accepts either SD or HD delivery.
• Require only one (fail if more) of the media file definitions above: exactly one rendition must match. Useful for delivery rules that forbid shipping multiple bitrates.

Two optional warnings catch common authoring mistakes:

• Discrepancies between declared and actual media file properties, for example a tag that claims 1920x1080 but ships a 1280x720 file
• Duration discrepancies between media files within the same VAST tag

Common issues this test catches

• Only one rendition shipped when the spec calls for three
• Low or high tier silently dropped from a re-encoded creative
• Declared bitrate in the VAST XML not matching the real file bitrate
• Correct bitrate but wrong codec or container
• Duration mismatch between renditions, which breaks adaptive playback
• API framework tagged wrong (VPAID declared, plain mp4 delivered, or the reverse)

How to evaluate your results

If a rendition requirement fails, the scan report shows which media files were found and which were missing. Send the failed report back to the creative team and ask for the missing bitrate tier to be added and re-encoded.

If discrepancies between declared and actual properties come up, the usual fix is regenerating the VAST XML from the final media files so the manifest reflects reality.

Related video tests

• Bitrate: measure the actual bitrate of each media file and compare it to your spec
• H.264 profile: main, high, or baseline profile checks
• Letterboxing: catch burned-in black bars from aspect ratio mismatches in each rendition

See every check on the video tests page.

Start validating your VAST renditions

Define the renditions your inventory requires, then run any VAST tag through Advalidation to confirm it delivers. Get in touch for a walk-through of how Required Media Files fits your QA workflow.