The loosely defined category of S1 MP3 players is comprised by a large amount of then-inexpensive handheld digital audio players. The players were mainly widespread around 2005–2006 but the series continued for years afterwards, blurring into that of so-called "MP4 players" employing S1 and competing architectures. Despite being primarily defined by the use of a system-on-a-chip of one of the Actions brands usually of the ATJ20xx series and some common core features, S1 products vary widely in software and hardware as well as design (including lookalikes and even counterfeits of other players), leading to a jumble of hard to identify internal designs, all employing non-mutually-interchangeable firmwares. Capabilities MP1, MP2, MP3 and WMA playback WMA DRM support on a small number of models Ogg Vorbis support on a small number of models mostly made after 2005, though this is not typically listed on the player's packaging and may come with drawbacks (such as lower maximum volume, decreased battery life or an inability to read ID tags). Acts as a USB mass storage device when connected to a computer: can be used as a general purpose flash drive, and does not require proprietary software to load music. Memory capacities between 64 MB and 32 GB. As with other forms of low-cost flash storage, some unscrupulous manufacturers resort to "flash fraud" - configuring the reported capacity to be higher than that of the actual storage, resulting in corruption due to wrap-around when the real size is exceeded in use. Powered by a AAA battery or a built-in rechargeable battery which charges via USB host connection. May be operable on USB power without batteries. 8-bit Z80 CPU core with on-chip Debug Support Unit (runs at typically 24.576 MHz, up to 60 MHz — software-controlled). 24-bit DSP with on-chip DSU. Firmware installation by in-system programming over USB, via the proprietary ADFU protocol. Some models have additional features, such as: FM radio (requiring an additional receiver, such as the Philips TEA5756). Graphic equalizer presets. Recording via built-in microphone or FM radio in ADPCM WAV (32 kbit/s mono or 64k bps stereo; using FFmpeg codec named adpcm_ima_wav), ACT (8 kbit/s) or VOR (32 or 8 kbit/s) formats, and ability to play back these files locally or via Windows Media Player when connected as an MSC device. Basic telephone number list viewer (using a proprietary database format). ID3 tag display. Ability to display lyrics in files with extension