This app was mentioned in 8 comments, with an average of 1.25 upvotes
I use this app to study the violin sounds, positions and even sighreading: https://play.google.com/store/apps/details?id=kochkapps.stringquartetsightread&hl=en
Don't translate in your head ("that's an E so my finger would go here", "that's a G so I'll play an open string"). Work on the written notes as finger placements directly. Associate the fingers with the notes, and practice your scales so it's just the finger pattern that changes based on the key. Practice pattern recognition with scale and arpeggio exercises, and random note reading with apps.
This app lets you do this away from your instrument as well. I curl my left fingers around the back of my phone so I can associate a given note with a finger.
Try sort of anchoring to the "home base" of open strings. Other than that, and practice in general, plain old flash cards/drills. Musictheory.net is great for this, and I also really like this app.
This app does exactly what you describe. There's a quiz mode that shows a random note in a chosen clef for a given instrument and listens for you to play it.
It does many other things too, it's pretty excellent.
I really liked this app when I was starting on alto clef. I just checked and the cello mode does have a tenor clef option. It has a simulated fingerboard which you can toggle the note visibility on. Pretty nifty and underrated app imo
On Android I use String Quartet which has an intonation sight-reading mode with customizable clef settings. I have the paid version, I don't remember what's unlocked with the free mode unfortunately.
Are you looking for something like this?
There's also an app store counterpart for the iPad.
Know that it will take a long, long time to sound not absolutely terrible. We're talking months to years of several hours of practice a day to be able to somewhat reliably produce a sort of okay tone, and that's with the guidance and feedback of a teacher.
Know that progress will be slow and be okay with it, and celebrate the tiny incremental successes. Don't rush to learn familiar pieces or vibrato and make sure you work to develop a really solid foundation so you can one day get to those things!
If you're cool with all that, it's an incredibly gratifying experience since there are so many things to work on and improve that there's always something you'll be able to see progress with. The coolest thing is struggling with practice, getting a good night's sleep, and the next day feeling how those neurons have really clicked together because of it to make it a little easier.
I've played many, many instruments and I can say from experience that it's the most difficult to produce a descent sound on-- besides viola!
Some things that improved my life and practice:
•This app got me started reading my clef and associating notation with fingering. •A stable music stand. Not a flimsy wire foldable one. Get one solid enough that you could use it for a writing desk, like a Manhasset (these are generally pretty easy to find second hand for not too much since city school music programs usually buy scads of them and people often... liberate them) •A teacher. This can be traditional paid lessons, a really, really patient friend who knows the instrument and is cool with giving feedback, or a supportive community group. Dedicated one on one lessons are generally best since you get more quality and more immediate feedback •Learn a little about the construction of the instrument and the physics of how it works! This equips you with some tools to find a good teacher, to adjust your setup with more confidence, and understand why "sucking" for a long time at the start isn't about you being bad at music, but the instrument being wholly unnatural, and entirely reliant on you for every minute aspect of its sound production. And it's just cool info!
Bonus: IMSLP is your new best friend. There are scales, exercises, old method books, pieces for all abilities, and all free because it's public domain.
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