The 500-series format is a standardized format for a modular signal processor system consisting of a combination of 500-series modules installed within a 500-series chassis (or rack), with the chassis providing power and audio connections for the individual modules. Originally invented by Automated Processes, Inc. co-founder Saul Walker, today numerous companies manufacture 500-series format products.
The modular nature of the 500-series format allows individual modules to be combined to create a customized signal processing chain, with individual modules added or changed as budget allows. Many 500-series chassis also offer portability, allowing a recording engineer to bring their desired signal processing chain to any recording session.