‘Semi-professional’ machines made their debut In the early 1970s.
These tape decks had features not found in consumer machines, and the mechanisms were usually designed to be more rugged and endure more frequent switching between functions. Semi-pro decks could range from two-channel (stereo) to 24-channel or 24-track machines. Multi channel decks could record one or more tracks at a time, while playing back other tracks already on the tape. This was important for musicians as they could record one or two instruments at a time, and have the ability to re-record an instrument or vocal track without erasing other tracks already recorded.
Semi-pro machines typically came in 7 ½ and 15 IPS formats, with some models having three speeds. Other models were single speed units designed to cut back on the costs associated with making a multi-speed model. All semi-pro multi-track models were designed to use the full width of the tape in one direction, so it was not possible to flip the tape over to the other side. If you did flip the tape over, you’d erase everything recorded on the first side.
Semi-pro machines may or may not have microphone level inputs as most home studios would use an external microphone mixer, and therefore would only have line level inputs and outputs. Some models used the standard consumer RCA connectors, while others used quarter phone (headphone) style connectors, where others used the professional balanced XLR connectors for inputs and outputs.
The most popular semi-pro brand (and heavily marketed to the home studio musician) was Teac/Tascam (Tascam being the pro division of Teac), Revox/Studer (Studer being the pro division), Otari, and Fostex. Fostex and Tascam made up to 16 track machines, and most decks with more than 4 tracks/channels on them would use half-inch or one-inch recording tape, compared to the consumer quarter-inch tape format.