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# System prepended metadata

title: How music NFTs are interpreting 721

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# How music NFTs are interpreting 721
We already talked about how ERC721 is badly designed and overengineered although it is still being used as primary smart contract standard for NFTs. The music NFTs are becoming more and more popular but the infrastructure is not so perfect. 
(what I think about music NFTs from infrastructure perspective)

Catalog
Catalog is using modified version ERC721 by Zora Protocol. The Zora Protocol maintains zero opinion about the structure of that metadata. It is using metadata schemas supported by JSON Schema which will generate Types, Parsers, Generators and Validators which will be served through ZDK.
https://github.com/catalogworks/media-metadata-schemas

Audius
Decentalised,community-owned and artist-controlled music sharing protocol.
If you’re interested in developing on Audius protocol, you need to run either one orr both Audius nodes for off-chain services. Discovery Node which indexes blockchains used by Audius protocol, serves API traffic and tracks usage metrics. Content Node hosts audio and image content and syncs this content across nodes to ensure data is consistent and highly available.
source: https://docs.audius.org/protocol/overview


Glass protocol

Decentralised video streaming platform, metadata are on arweave
https://glass.xyz/
Contracts https://github.com/GlassProtocol/hyperlink

Cargo 
https://cargo.build/
Supports: mp3,mp4, webm, glb, gltf files